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Lamprocyrtis nigriniae (Caulet, 1971)

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Benson, 1966, p. 479-482; pl. 33, figs. 2-3:

Conarachnium sp.

Cephalis cylindrical, generally open at the top but closed in a few specimens, with a corona of 1-8 or more spines surrounding the opening, with a straight dorsal face and a pair of dorso-lateral lobes. Apical bar straight, free within the cephalis proximally, a straight dorsal rib in the cephalis distally, extends as and is collinear with a three-bladed apical spine or horn, the two lateral blades of which merge with the dorsal face of the cephalis. Vertical bar extends as a short, thin, conical, vertical spine originating at the base of the ventral face of the cephalis. Cephalis separated from a broad, slightly inflated, conical thorax with irregular outline by a change in contour but not by a distinct stricture. Terminal margin of thorax incomplete, but in one specimen the mouth slightly constricted but its margin incompletely developed. Surface of both joints smooth in a few specimens but generally with a few scattered thorns or short spines. Pores of the cephalis circular, subequal, subregularly arranged, slightly wider than the intervening bars. Pores of the thorax subcircular to polygonal, increase in size distally (from about 4-9 Ám proximally to 18-20 Ám distally), with subregular hexagonal arrangement in longitudinal rows, 7-9 on half the thoracic circumference. Four collar pores present, but in a few specimens a pair of jugular pores are present and are covered by the dorso-lateral lobes. Secondary lateral bars do not extend as spines. Dorsal and primary lateral bars extend as ribs in the thoracic wall for a distance of 2 to 4 thoracic pores; ribs occupy slight furrows in the thorax and generally terminate in short, inconspicuous spines.

Measurements; based on 30 specimens from stations 27, 34, 46, 56, and 106: length of cephalis 25-46 Ám, of thorax 76-172 Ám; breadth of cephalis 25-32 Ám, of thorax 86-138 Ám; length of apical born 18-37 Ám, of vertical spine 2-6 Ám, of coronal spines surrounding the open top of the cephalis 5-12 Ám, of dorsal and primary lateral spines 5-12 Ám.

Remarks. This species is distinguished by the opening in the upper face of the cephalis, by the broad thorax with irregular outline and incomplete terminal margin, and the absence of terminal or subterminal spines. The basic structure of the cephalis and apical horn, including the dorso-lateral lobes is the same as that of Anthocyrtium oxycephalis , Anthocyrtidium cineraria , and Lamprocyclas maritalis.
No reference to this species was found in the literature. The only illustrated species having a cephalis with a straight dorsal face and with its top open and lacking an abdomen is Sethoconus conicus Popofsky (1909, p. 287, Pl. 33, figs. 10, 11). This species differs from the Gulf species in the presence of a latticed apical horn, the lack of a corona of spines surrounding the opening of the cephalis, and in having a smooth surface with small pores separated by wide intervening bars. Campbell (1954, p. D128) indicates that Sethoconus Haeckel (1882, p. 430) is an objective synonym of Conarachnium Haeckel (1882, p. 430) [= Ceratocyrtis BŘtschli, 1882a, p. 536]. The Gulf species conforms to the definition of Conarachnium Haeckel, but because no specimens were observed with the terminal margin of the thorax completely developed, the complete range of variation of this species could not be determined. For this reason a new name is not proposed for this taxon.

Distribution. This species is cosmopolitan in the Gulf, occurring as far north as stations 191 and 192. It is absent at the marginal stations 90 and 130 and the northern shelf localities including station 194 and those north of it; therefore, it prefers an offshore, more nearly oceanic habitat. It is common at stations 91 (2.4%), 99 (2.4%), 106 (3.2%), 115 (20%), and 151 (2.6%) and rare to nearly common at most of the other stations in the Gulf. Across the Gulf from stations 91 to 99, it has a greater frequency at stations 91 and 99 than at 92 and 93 (1.6%) and 95 (0.2%). It, therefore, responds to upwelling along both the coasts of Baja California (station 91) and the Mexican mainland (station 99) in this region of the Gulf. South of this area its frequency is lower but reaches 1.6% at stations 46 and 56, the latter located within a region of upwelling. It is very rare at station 64 which is in an area of upwelling off the Mexican mainland. Its average frequency in the northern half of the Gulf is much greater than that in the southern half. It is nearly common to common at all stations within the diatomite facies except 191 and 192 where it is very rare; therefore, its distribution in this region is controlled by upwelling.
Benson 1966


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