728 SYSTEMATIC PALEONTOLOGY 



" The dimensions of a specimen of average size are : Length 7 mm. , 

 height 4.5 mm., greatest thickness 4.8 mm. 



" The name Pholas cretacea was originally applied to a group of casts 

 of the tubes of one of the Pholadidce, without any knowledge of the shell 

 characters. At a later date the original author of the species described a 

 single individual of a shell and referred it to the same species as the pre- 

 viously described tubes, ' because it is of about the proper size to form 

 such tubes.' In themselves, the tubes of this group of pelecypods posses? 

 no characters which can be used for specific determination, and conse- 

 quently the species Pholas cretacea, afterwards referred to the genus 

 Martesia, may be considered as founded upon the shell described by Gabb. 

 Whit-field has illustrated Gabb's specimen and redescribed it, but he saw 

 no additional specimens. In the recent collections of the Survey, fifty 01 

 more individuals of this species have been observed in a fragment of fossil 

 wood from 1 to 1 in. in diameter and 8 in. long. The entire surface 

 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 characters, but a comparison with Gabb's 

 type of M. cretacea has shown them to be not essentially different from 

 that species. Some of the examples are shorter than usual and conse- 

 quently taper more abruptly to the posterior extremity than the average 

 form, but the most important variation is the presence or absence of the 

 supplementary oblique furrow in front of the primary one extending 

 from the beak to the ventral margin. In the majority of individuals this 

 furroAV is present and its absence is more apt to be a feature of the smaller 

 and presumably the younger shells. In a few specimens of nearly maxi- 

 mum size this furrow is nearly obsolete, being noticeable only near the 

 ventral margin, and in one specimen it is absent from one valve, although 

 faintly indicated on the other." Weller, 1907. 



The species has a doubtful representation within the Delaware-Maryland 

 area. A single somewhat distorted tube referred tentatively to this 

 species was collected along the canal. A cast of the interior with frag- 

 ments of the substance still adhering has even more dubious affinities. 

 The surface characters are apparently very similar to those of M. cretacea, 



