614 SYSTEMATIC PALEONTOLOGY 



Although the representatives of this species differ so widely in general 

 aspect, still there seems to be no reason to consider these differences as 

 more than individual mutations of a variable species in a variable genus. 

 The ratio between the number of costals and their prominence is very 

 constant, those individuals in which the primaries are few in number 

 being very heavily sculptured. 



Anomia forleplicata is a more solid shell than A. ornata Gabb, with a 

 much more vigorous radial sculpture and a relatively more feeble con- 

 centric. 



Occurrence. MONMOUTH FORMATION. Brightseat, 1 mile west of 

 Friendly, McNeys Corners, Prince George's County. 



Collection. Maryland Geological Survey. 



D. Dysodonta 



Superfamily MYT1LACEA 

 Family MYTILIDAE 



Genus MODIOLUS Lamarck 

 [Prodr. Nouv. Class. Coq., 1799, p. 871 



Type. Mytilus modiolus Linne. 



Shell equivalve, inequilateral, transversely or obliquely ovate in outline ; 

 ligament external, opisthodetic ; hinge edentulous ; anterior muscle impres- 

 sion atrophied; pallial line simple. 



The genus is separated from Mytilus by the character of the beaks which 

 are non-terminal, wider and rounded anteriorly. It has a long geologic 

 range, at least from the beginning of the Mesozoic and possibly from the 

 Devonian. The recent species are about seventy in number and are most 

 abundant in the tropical seas. Unlike Mytilus, the representatives of 

 Modiolus are nest-builders and burrow or spin a woven structure from 

 stones and fragments of shells. 



A. Latitude of adult shell exceeding 20 mm. 



1. Shell obtusely angulated at the posterior dorsal extremity. 



Modiolus burlingtonensis 



2. Shell smoothly rounded at the posterior dorsal extremity. 



Modiolus sedesclarus 



B. Latitude of adult shell not exceeding 20 mm Modiolus trigona 



Etymology: Modiolus, small drinking vase. 



