1902.] NATURAL SCIENCES OF PHILADELPHIA. 697 



the Condylocardiidce the resilium is immersed and the ligament 

 feeble or obsolete. 



The hinge-formula, when fully developed, as in Carditamera, 

 is R 10 10101 10 ' ^^^^ ^^ many forms the laterals are obsolete and the 

 distal cardinals very feeble, coalescent with valve-margin or nymph, 

 and hardly to be made out, so that in such forms as Venericardia 

 the formula may be reduced to ^ q "oio" ^- The hinge has never 

 more than two left cardinals, the posterior one invariably long- 

 drawn-out, a characteristic feature of the family, while the Veiie- 

 ridce never have less than three left cardinals, so that convergent 

 forms may readily be referred to their proper family. The teeth 

 are usually finely striated. It is evident that with part of the hinge 

 armature so liable to degeneration too much stress in classification 

 must not be laid on such mutable features, and as a matter of fact 

 the generally accepted subgeneric and sectional groups are chiefly 

 based on external form, a character which proves unexpectedly 

 constant when the groups are traced back through the line of their 

 fossil progenitors. 



The lunule is usually small, or even obsolete, and frequently 

 unequally distributed between the valves, but when present is 

 usually circumscribed by a deep, narrow sulcus, the termination of 

 which on the inner left hinge-margin is frequently marked by a 

 small but distinct pustule received into a dimple in the opposite 

 valve ; this pustule may or may not coincide with the left anterior 

 lateral lamina. In Beg ulna the sulcus is so extended by the torsion 

 of the hinge as to become tubular, though this perforation is prob- 

 ably closed by an organic plug in the living shell. The escutcheon 

 is frequently linear or obsolete; when present it is usually limited 

 by a ridge or keel. The pallial line is almost invariably entire, 

 but in Cardiocardita ajar there is a broad posterior scar in front of 

 the posterior adductor which simulates an indentation of the line 

 and is probably caused by some enlargement of the siphonal 

 muscles. 



The valves are usually white within and the color of tlie exterior 

 is dull, except in a few tropical species. 



The group is of ancient origin and has Mesozoic representatives, 

 but only those of Tertiary and Recent horizons will be considered 

 here. 



