MUSEUM OF COMPARATIVE ZOOLOGY. 449 



typical form of the fromis in tlio. absence of tlic sn])OTTiciiil granulations, and in 

 the presence of a deep and .strong pallial Hinns, which charactern indicate tliat 

 it should form a special section of the group. Tiie hinge is also remarkably 

 coarse and strong, Jt was dredged by the Albatross off the coast of Ecuador, 

 in 741 fms., and externally presents much the a})pearance of a large high speci- 

 men of Mactra lateralis. 



In the type of Poromya the pallial sinus is almost obsolete; its retractor 

 muscles are either mainly incorporated in the septum, the muscular contrac- 

 tions of which serve to move the siphons, or they are replaced by the septal 

 muscles. In the present species, however, there is a large and strong pallial 

 sinus with its usual muscles, and the septum is consequently only very slightly 

 furnished with muscular fibres, and does not serve to retract the siphons. 

 The valve to the branchial siphon is large, and the palps are enormous. The 

 anterior edges of the anterior palps are notched or papillose toward the median 

 line, a condition not observed in the other species. The foot is pointed and 

 slightly geniculate. There are seven anterior, and eight or nine posterior 

 gill lamellae; the two areas are rather narrow, and their ends closely approach 

 one another near the middle of the foot on each side. In front of the ridge 

 which precedes the large branchial valve, and between it and the foot, are 

 four or five quite prominent elevations of the surface closely resembling the 

 branchial lamellaj, but with their length in the axial direction of the animal. 

 There are no fissures between these, but they seem very like branchial lamellae 

 in process of development. Both the longitudinal branchial areas on each 

 side are fissured, and their blood-vessels reach them from behind. 



General Considerations on the Nature of the Septicm in Poromyidce and Cuspi- 

 dariidcB. — The facts above stated indicate that the septum in these groups is 

 essentially a prolongation forward, and a specialization of the ordinary si- 

 phonal septum. The septum, as pointed out in Cardium, may be so prolonged, 

 while the normal gills are fully developed and unconnected with it. In Ver- 

 ticordia it may be so prolonged, and may have acquired a conspicuously fleshy 

 texture, without fissures, while the gills lie prone upon it, more or less adnate. 

 The muscular apparatus by which the siphons are retracted, and whose normal 

 points of origin are at the side of the ordinary septum, appear to be shifted 

 to its surface; different species show this process in different stages of pro- 

 gress, and in the only case among the Poromyas where the fibres follow the 

 normal direction in other Pelecypods, the septum is destitute of the muscular 

 structure which is so prominent in the other Poromyas. In the specialization 

 of the septum the musculation develops from behind ; when branchial lamina; 

 are situated upon the septum, and are not simply the ordinary ctenidia in an 

 adnate condition, the addition of a second series is made at the posterior end, 

 and all the branchial areas appear to receive their blood supply from behind. 



There is not a particle of evidence to prove that the septal branchial lamellse 

 of Poromya and Cetoconcha are homologous with the ctenidia of Verticordia, 

 L7jonsiella, Perna, or Cardium. The fact that Cuspidaria has neither ctenidia 

 nor any specialized laminae on the septum lends probability to the assumption 



VOL. XVIII. 29 



