MARYLAND GEOLOGICAL SURVEY 719 



There are a number of valves of uncertain relationships occurring with 

 C. subradiata and at nearby localities. Their affinities are undoubtedly 

 with subradiata, but whether they are properly referable to the same 

 species, or subspecies or an entirely distinct species, only the collection 

 of further material will establish. They differ from the type in the greater 

 compression of the valves and the consequently more obtuse posterior keel. 

 Some are more sharply sculptured, others less sharply, but there is an 

 aspect of consanguinity about the group that makes it seem probable that 

 they are isolated representatives of an unbroken series. 



Occurrence. MONMOUTH FORMATION. Brightseat, Brooks estate, near 

 Seat Pleasant, ? McNeys Corners, ? 1 mile west of Friendly, Prince 

 George's County. 



Collection. Maryland Geological Survey. 



Family SAXICAVIDAE 



Genus PANOPE Menard 

 [Me"m. Nouveau Gen. Coq. Biv., 1807, p. 31] 



Type. Panope aldrovandi Menard. 



Shell equivalve, oblong, gaping at both ends; surface smooth or con- 

 centrically furrowed ; ligament external, conspicuous ; a single prominent 

 conical tooth in each valve; pallial sinus deep. 



A genus that has been in existence since the close of the Cretaceous, cul- 

 minated in the Tertiary and is represented to-day by about a dozen species 

 occurring chiefly in cooler waters. 



Dall has given the following discussion of the genus : 



" This well-known genus, after the exclusion of the Saxicavoid species, 

 forms a very natural group, related to the Myacidce on the one hand and 

 to Saxicava on the other. Some pearly forms formerly confounded with 

 it have long been eliminated, and have relations, no doubt, with +ina- 

 tinacea. 



" T have had the advantage of an opportunity to study several Pacific 

 Coast forms in life and in -their natural surrounding, as well as a very 



Etymology: Panopea, a sea-nymph. 



