S3 6 TRANSACTIONS OF THE WAGNER FREE 



detic; resilium subinternal below the ligament, with a small lithodesma ; area 

 obscure or absent; a depressed false lunulein front of the beaks; valves nearly 

 or quite closed ; pallial sinus small or obsolete. 

 Cretaceous to recent fauna. 



Ex. LiopiMa, Cymel/a, Psilomya, Poromya, Dermalomya, Cetomya, Cetoconcha, f Bas- 

 terotia. 



The septum in this group is of compound origin, but in what proportion 

 the original gills contribute to it remains to be determined. 



FAMILY CUSPIDARIIDyE. 



Ventricle bilobate, embracing the rectum or dorsal to it, with two aortas; 

 gills obsolete or absent, represented only by the ciliated foramina in a muscu- 

 lar septum; palps degenerate or obsolete; foot small, digitiform, grooved, with 

 a minute byssus but no opisthopodium ; siphons united, long or short, with a 

 few papilla: and obsolete curtain valve ; monoecious, marine. 



Shell subequivalve, rostrate, earthy or cellulo-crystalline, rarely with sur- 

 face granulations ; hinge edentulous or with subumbonal desmodont tubercula- 

 tion, sometimes buttressed ; ligament subinternal, anterior to the beaks or ob- 

 solete ; resilium internal with a mesial or ventral lithodesma ; arcaamphidetic or 

 obscure ; valves closed except at the tip of the rostrum ; pallial line simple, the 

 retraction of the siphons usually effected by the contraction of the septum, 

 which latter leaves a scar on the valves resembling a pallial sinus. 



Jurassic to recent fauna. 



Ex. Cuspidaria, Cardiomya, Leiomya, Luzonia, Plectodon, Tropidomya, ? Plectomya, Hal- 

 onympha, Myonera, etc. 



In the larger species of Cuspidaria, the ligament as distinguished from the 

 resilium is entirely prosodetic or anterior to the beaks, and fairly well devel- 

 oped ; in most of the species it is obsolete or absent, but always anterior when 

 present. It is the only instance I have observed where a ligament, not amphi- 

 detic, is anterior. 



Led by observations on Lophocardhtm and Verticordia, in which the sep- 

 tum is obviously a forward extension of the siphonal septum, I have heretofore 

 claimed a similar origin for the septum of Cuspidaria. The researches of 

 Grobben have led him to the conclusion that it is branchial in its origin, as 

 claimed by Pelseneer. The gills, as shown by embryology, are originally cutic- 

 ular proliferations from the inner surface of the mantle. Lyonsiella (alaskana} 

 and Halicardia show curious inwardly projecting laminae from the mantle 

 taking part in the formation of their septa. Now if the respiratory lamellae be- 

 come obsolete and the fundamental mantle surface from which they originally 

 sprang becomes modified into a muscular organ with special functions, this 

 latter can only be said to be derived from the gills in a sense which ignores 

 the origin of the gills. 



