112 PtECILASMA EBURNEA. 



Affinities. — In the structure of the carina, and more 

 especially of the scuta, there is a strong affinity between 

 the present and following species ; for we shall imme- 

 diately see that in P. eburnea there is evidence of the 

 scuta being composed of two segments fused together ; 

 and the larger segment is furnished with an internal 

 oblique, strong, basal rim. To this same species there is 

 an evident affinity in the form of the mandibles and of the 

 caudal appendages, and in the anterior rami of the cirri 

 being longer than the posterior rami. Notwithstanding 

 these points of affinity, I consider that P. fissa is more 

 closely related in its whole organisation, as more par- 

 ticularly shown in the arrangement of the spines on the 

 cirri and in the presence of terga, to P. crassa than to 

 P. eburnea. Although in Dichelaspis, the scuta are 

 invariably composed of two almost separate segments, 

 yet P. fissa shows no special affinity to this genus. 



5. P^CILASMA EBURNEA. PI. II, Fig. 5. 



Trilasmis eburnea. Hinds. Voyage of Sulphur, 1844, vol. i, 

 Mollusca, PI. xxi, fig. 5. 



P. valvis 3 ; scutis acumi?iatis, ovatis; ad pedunculum 

 pane transverse spectantibus ; dentibus internis umbo- 

 nalibus fortibus : tergis nullis : carina termino basali in 

 discum amplum oblongum infossum producto. 



Valves 3 ; scuta pointed, oval, placed almost trans- 

 versely to the peduncle ; internal umbonal teeth strong : 

 terga absent : carina with the basal end produced into a 

 large, oblong, imbedded disc. 



Spines on the upper segments of the posterior cirri, 

 arranged in three or four approximate longitudinal rows, 

 making small brushes. 



Habitat. — New Guinea, attached to the spines of a dead Echinus. Brit. 

 Mus., and Cuming. 



General Appearance. — Capitulum flat, pear-shaped, 



