INSTITUTE OF SCIENCE, PHILADELPHIA. 529 



ment amphidetic, subinternal ; armature of the hinge obsolete or feebly lamel- 

 lar ; adductor scars subequal, pedal well marked ; gills as in Mytibis, the outer 

 serving as an incubatory sac for the nepionic young, as in Unio ; mantle lobes 

 united to form siphonal and pedal orifices ; foot elongate, thick in front, at- 

 tenuated distally ; the nepionic young ovate, with a long, straight hinge-line. 



Tertiary to recent fauna. 

 E,x. Modiolarca^ Phaseolicania, Kidderia. 



In Modiolarca there is a minute but distinct impression in front of the 

 beaks, visible best on the edge of the separated valves. The nepionic stage 

 recalls that of Solemya, but is more ovate. 



Family PRASINID^? 



Shell modioliform, small, equivalve, free, with a narrow byssal gape, area 

 obscure or none ; a depressed space in front of the beaks ; ligament subinter- 

 nal, opisthodetic ? anterior adductor scar absent ? posterior adductor scar low 

 and centrally situated ; soft parts unknown. 



Eocene to recent fauna. 

 Ex. Prasina, Julia, Berthelinia. 



The scars are very difficult to make out on these small polished shells 

 (Stoliczka), and it may well be that an anterior adductor exists, in which case 

 the group would be merged in the family Modiolarcidce. The " teeth " are 

 lamellar extensions of the margin of the valve below the impressed area. 

 Anomaloinya Cossman, which has been placed in this family, strongly recalls 

 Dacrydiuin. 



Order ANOMALODESMACEA. 



Although this group in the general classification has usually been placed 

 after the forms here included in the Teleodesmacea, no linear arrangement can 

 properly express their natural relations. This and the preceding order repre- 

 sent for us lines of parallel development of the most ancient origin, while the 

 Teleodesmacea probably took their rise chiefly from forms which, genetically, 

 would be included in the present order. The modern Anomalodesmacea are 

 nearly all much specialized and their archaic characters veiled by later adapta- 

 tions. That any have come down to us retaining indications of their archaic 

 character is probably due to the burrowing habit, which has shielded them from 

 many vicissitudes and checked in this way the perfecting processes of selec- 

 tion. 



The student will understand, therefore, that the present group is here re- 

 garded as having one general origin with the Teleodesmacea, the two being 

 contrasted with the Prionodesmacea, the latter differing more from either than 

 the former differ from each other. Part of the original stock has produced the 

 perfected types of Pelecypods, which have lost all their archaic characters and 



