140 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



Placentula, Jones and Hell. 



"Valves suborbicular, nearly semicircular on the 

 ventral border, straight on the dorsal margin inside, 

 but projecting with unequal and variable angles at 

 the outer dorsal region. Surface flat or slightly- 

 convex, surrounded by a raised rim, which slopes 

 down suddenly outside to the edge of the valve. 

 This rim encloses a depressed and reticulated area, 

 and in or near the antero-dorsal region there is a 

 small depression defined by a raised loop-like boi'der. 

 "Within certain bounds all these features are variable 

 in different individuals." 



(Lat. Placentula, a little cake.) 



Octonaria, Jones. 

 "In this peculiar form of carapace the valves are 

 Rubovate in outline, thick, and flattened, with 

 variously moulded surface. The ventral margin is 

 usually straighter than the dorsal. The superficial 

 plateau is more or less regularly bordered by a 

 curved ridge, within which there are one or more 

 hollows and different elevations ; the latter are 

 usually connected with the ridge, but sometimes 

 isolated. The inclination showni by the ridge in 

 many cases to turn in on itself in the ventral 

 region, and thus more or less closely imitate a 

 figure-of-eight, is the foundation for the generic 

 name. In all cases the edge view of the carapace 

 is much like that of Thlipsura corpulenta, that is, 

 suboblong with subacute ends, the valves being 

 nearly flat on the surface, and sloping down to the 

 edges all round." 



On some of the Silurian Beyrichicc, viz., B. tuber- 

 culata var. gibbosa, Reuter, B. Kloedeni var. granulata, 

 Jones, B. K. var. intermedia, Jones, B. K. var. 

 subtorosa, Jones, B. K. var. tuberculata, Salter, B. K. 

 var. torosa, Jones, and B. Maccoyiana, Jones, there 

 is a large lobe, called the " hypertrophied lobe," 

 developed towards one end of the ventral region of 

 each valve. They have been ascribed to advanced 



