470 FREDERICK W. SARDESON 



consequent asymmetrical visceral structure is seen in all Recent 

 Gastropoda, including those which now have simple conical 

 shells and the families to which the simplest Cambrian shells 

 are referable, these appear, because of their visceral asymmetry, 

 to have descended in turn from ancestors which had spirals or 

 coils. Some Recent cones are indeed traceable by fossils to 

 coiled ancestors. The larva, too, of many Recent species has a 

 curved or coiled protoconch or larval shell, and this has been 

 taken as evidence or as proof that the gastropods with conical 

 shells had ancestors with spiral shells.' Conical shells may 

 therefore be either the ancestral cones or again the last derived 

 stage in their evolution. One is led finally by the evidence of 

 continual series, apparently, of conical shells from Cambrian to 

 Recent time, to the conclusion that, though they seem primitive, 

 the Cambrian shells are not more so than Recent ones — a con- 

 clusion, indeed, which it is difficult to accept in view of the very 

 primitive aspect of the whole Cambrian gastropod fauna ; and 

 first of all therefore I desire to emend the theory of the relation 

 of the spiral shell to the visceral asymmetry. 



ORIGIN OF THE SPIRAL SHELL. 



Known Cambrian fossil Gastropoda are referable to Order I, 



Prosobranchia, excepting provisionally the Hyolithoidea. 



Regarding the range of the Prosobranchia, Suborder i, Dioto- 



cardia (Aspidobranchia) includes the group Docoglossa, to which 



belong Scenella and Patella with conical shells, and these 



together range from Cambrian to Recent time, without known 



coiled or spiral ancestor. The group Rhipidoglossa includes 



some Recent conical shells, Fissurella and Fissuridea, which 



together range from Carboniferous to Recent, and whose only 



earlier representatives are coiled, such as Bellerophon and 



Salpingostoma of Upper Cambrian and Ordovician to Triassic. 



Suborder 2, Monotocardia (Pectinibranchina), has also simple 



conical-shaped species, the Capulus of Recent time, and Steno- 



theca and Platyceras which range from the Cambrian upwards. 



Of these the earlier ones especially have a short coiled apex. 



'See recently, A. W. Grabau, A7nerican Naturalist, Vol. XXXVI (1902), 

 p. 918. 



