446 STUART WELLER 



slightly while becoming stronger anteriorly, finally passing into 

 the bases of the crura ; shortly in front of the point of origin of the 

 crural ridges on the hinge-plate the socket-plates are rapidly 

 reduced in height and soon become obsolete, beyond which point 

 the hinge-plate is not connected with the inner surface of the 

 valve, but becomes a concave plate joining the bases of the crura 

 and terminating anteriorly in a short distance. The complete 

 form of the brachidium is not known, but it is probably short, 

 not reaching the mid-length of the valve. 



Remarks. — -This genus is perhaps most closely allied to Cranaena, 

 from which it differs in the extreme concavity of the hinge-plate, 

 the cavity between it and the inner surface of the valve being 

 reduced in height, and in the absence of the perforation of the 



Fig. 6. — A series of nine cross-sections (X25) of the rostral portion of the brachial 

 valve of Hamburgia typa, n. sp. 



hinge-plate at the apex, which is, perhaps, the most diagnostic 

 character. The genus is totally distinct from Dielasma, in which the 

 crural plates originate as ridges upon the inner surface of the valve 

 instead of upon the concave surface of the hinge-plate. The concave, 

 transverse plate between the bases of the crura is somewhat similar 

 in the two genera except that it is not connected along its median 

 line to the inner surface of the valve in Hamburgia, but in 

 Dielasma the inner surface of this plate furnishes attachment for 

 the adductor muscles, which is apparently not true in Hamburgia. 

 Genotype. — H. typa, n. sp., Hamburg oolitic limestone of 

 Kinderhood age, Hamburg, 111. 



Dielasmella n. gen. 

 Description. — Shell terebratuliform, compressed. Pedicle valve 

 with well-developed dental lamellae of moderate length. Brachial 

 valve without median septum or true hinge-plate, the socket- 



