10 AMERICAN JOURNAL 



dorsal margin sloping a little posteriorly from the beaks. Sur- 

 face ornamented with radiating costse, which are slender on the 

 anterior half of the space in front of the linear furrow, extending 

 obliquely backward and downward from the beaks to near the 

 middle of the base of each valve, but stronger and more widely 

 separated between this and the furrow ; while behind the latter 

 they are very oblique, less strongly defined, and about as widely 

 separated as on the anterior region ; small regular ridges and 

 furrows also mark the valves concentrically. 



Length 1-16 inches ; height and convexity each about 0-66 

 inch. 



The only specimen of this shell yet brought in, is mainly a 

 cast in brown ferruginous matter. It seems to be a little inequi- 

 valve, the posterior ventral margin of the right valve projecting 

 slightly beyond the other, but this may be due to distortion, al- 

 though there would appear to have been no displacement of the 

 upper margins of the valves. A cast of the posterior adductor 

 muscle, is seen to be narrow ovate and situated near the cardinal 

 margin, about one-third the length of the valves from the poste- 

 rior end. The gap of the anterior margins does not result from 

 a notch or distinct truncation, and seems not to have been very 

 wide at any stage of growth. A slight portion of the anterior 

 margins of the valves, near the beaks, having been broken away 

 in the specimen, their exact character there cannot be well made 

 out, but they seem to have been a little rellexed. Between the 

 beaks some remaining portions of the matrix adheres, and in 

 this I have thought I could see some indications of a broken ac- 

 cessory valve, but no traces of its form can be determined. 



Of course, without better specimens it is scarcely possible to 

 determine, beyond doubt, whether or not this shell really belongs 

 to the existing genus Martesia. It is more probable that if we 

 knew all of its characters it would be found to belong to a dis- 

 tinct group. Mr. Tryon, who is well known to be an excellent 

 authority on the Pholadacea, thought, on examining the speci- 

 men, that from all of its characters that can be made out, it will 

 probably be found to be nearly related to Martesia. 



The specific name is given in honor of Mr. A. R. Roessler, 

 of the Geological Museum of the General Land Office at Wash- 

 ington City, to whom I am in indebted for the type specimen. 



Locality and position. The specimen was brought by Mr. 

 Roessler from near Fort Belknap, Texas, where he says it was 

 found in a formation containing fossil wood replaced by oxyd of 

 copper, and believed by him to be of Permian age. It certainly 

 has a more modern look than any palaeozoic shell I have ever 

 before seen. 



