244 



STUDIES IN EVOLUTION 



NEOTREMATA. 

 (ve<k young, and rprjua perforation.) 



(Plate XI, figures 5-12.) 



Protegulum as in the preceding order in primitive forms, 

 becoming more circular and with shorter and more arcuate 

 hinge in the pedicle valve of derived types. Growth of the 

 dorsal valve tending to become peripheral. In the opposite 

 valve the pedicle more or less surrounded by progressive 

 neanic growth posterior to the initial hinge. Pedicle fissure 

 remaining open in primitive mature forms, becoming enclosed 

 in secondary forms during neanic stages, and in derived 

 tj^es enclosed in early neanic or nepionic stages. Valves 

 inarticulate. 



Including the genera : 



Anr.istocrania. 

 Acrothele. 

 Acrotreta. 

 *Conotreta. 



* Crania. 

 *CranieUa. 



Craniscus. 

 *Discina. 



* Discinisca. 



*Discinopsis. 

 Helmersenia. 

 Kayserlingia. 

 Lindstrcemella. 



* Linnarssonia. 

 Mesotreta. 



*CEhleHella. 



* Orbiculoidea. 



PJiolidops. 

 Pseudocrania. 



* Rcemerella. 

 *Sc7iizambon. 



* ScMzoholus. 



* ScMzocranki. 

 Siphonotreta. 



*Tre7natis. 



PROTREMATA. 



(tt/ow early, and Tpijfia perforation.) 



(Plate XI, figures 13-21.) 



Protegulum of the dorsal valve as in the Atremata. In 

 the ventral valve it has become modified to an elliptical or 

 circular form with arcuate hinge. Pedicle enclosed in early 

 nepionic stages by a pro-deltidium ; posterior covering (deltid- 

 ium) retained at maturity, or resorbed or abraded in neanic 

 stages, so that the pedicle protrudes between the two valves. 

 Valves articulate. 



