Description of a Species of Caligus. 229 



Viewed as a whole, the cephalo-thoracic segment is shghtly con- 

 vex, and has an obtuse ovate form, a Uttle broader posteriorly, 

 with an emargination in front, (A, fig. 7,) and a deep sinus on 

 each side in the posterior margin, (B.) It is bounded, both ante- 

 riorly and laterally, by a thin transparent margin, which appears 

 transversely striated, when highly magnified. The lateral mar- 

 gin is about four times as wdde as the anterior. A row of ex- 

 tremely minute curved spines project above the junction of the 

 membranous margin, as is exhibited in figs. 1 and 19. Similar 

 spines are scattered over the back ; but a very high magnifying 

 power and the most favorable light are required to discover them. 



The articulation of the first with the second cephahc segment, 

 though mostly imperfect, approaches a perfect joint towards each 

 side, (C, fig. 7,) where there is an osseous process in the two seg- 

 ments, with opposite articulating surfaces. The process on the 

 first segment is narrow, and transversely oblong. That on the 

 second is long and slender, and extends to a point laterally in ad- 

 vance of the eyes ; it is much enlarged at the articulation, and at 

 that place resembles the process on the anterior segment. 



The articulation of the cephalic with the thoracic portion of 

 the cephalo-thoracic segment of the body is curved parallel with 

 the anterior margin of the animal, and terminates on each side, 

 near the centre of each lateral half of the cephalo-thoracic seg- 

 ment, (D, fig. 7.) From this point the articulation of the epime- 

 ral with the central segments commences. A thin semi-corneous 

 margin extends from the cephalic segment, and covers its articu- 

 lation with the adjacent parts. The junction of the epimeral and 

 cephalic segments is directed towards the anterior portion of the 

 lateral margin, but becomes obliterated before reaching it ; the 

 junction with the thoracic segment is continuous in a curve, con- 

 cave inward, to the posterior margin just outside of the sinuses 

 in the latter segment. An osseous articulation, similar to that be- 

 tween the two cephalic segments, unites the epimeral and ce- 

 phahc segments, (E, fig. 7;) the process on the former is long 

 and slender, and curves backward, giving firmness to that portion 

 of the shell. 



The thoracic portion of the cephalo-thoracic segment approxi- 

 mates to a circular form. The sinuses before referred to, (B, fig. 

 7, ) are situated in its outer posterior margin. Between each sinus 

 and the articulation of the epimeral segment, there is a narrow 



