MOLLUSCA. 655 



of the hinge-line into the straight basal margin, posterior margin 

 subtruncate, post-cardinal margin sloping backward from the 

 posterior extremity of the hinge-line. Surfac^e of each valve 

 marked by a deep, narrow groove, extending from the beak 

 obliquely backward tO' the ventral margin which it meets in 

 front of the middle of the shell; in most individuals a second 

 groove close tO' and parallel with the first, but a little wider and 

 shallower, is introduced a short distance below the beak and 

 continues to* the margin. The anterior region oi the shell is 

 marked by fine costse which bend abruptly upward in front of the 

 oblique grooves., continuing to. above the middle of the shell, 

 where they make a nearly rectangular turn and continue in a 

 horizontal direction to the anterior margin, surrounding two 

 sides of, and sharply differentiating, a smooth, triangular, slightly 

 raised area in the antero-ventral region of each valve. The pos- 

 terior region of the shell is marked by broader, rounded costs, 

 parallel with the margin of the valves. 



The dimensions oi a specimen of average size are : length, 7 

 mm.; height, 4.5 mm.; greatest thickness, 4.8 mm. 



Remarks. — The name Photos' cretacea was originally applied 

 to a group of casts of the tubes of one of the Pholadidae, without 

 any knowledge of the shell characters. At a later date the orig- 

 inal-author of the species described a single individual of a shell 

 and referred it to the same species as the previously described 

 tubes "because it is of about the proper size to form such tubes." 

 In themselves, the tubes of this group of pelecypods possess fto 

 characters which can be used for specific determination, and con- 

 sequently the species Pholas cretacea, afterwards referred to the 

 genus Masrtesia, may be considered as founded upon the shell 

 described by Gabb. Whitfield has illustrated Gabb's specimen 

 and redescribed it, but he saw no additional specimens. In the 

 recent collections of the Survey 50 or more individuals of this 

 species have been observed in a fragment of fossil wood from 

 I to Ji^ inches in diameter and 8 inches long. The entire sur- 

 face of this wood is filled with' the burrows of this species, and 

 in each burrow is a well preserved shell or the internal cast of a 

 shell. These specimens show some variation in several charac- 



