STANTON.] AVICULID^E. 75 



straight, about as long as the greatest anteroposterior diameter, and 

 forming an angle of 50° to 70° with the umbonal ridge; hinge area com- 

 paratively narrow, with five or six (or possibly more) large cartilage 

 pits, each of which is nearly twice as broad as the space between them; 

 posterior margin nearly straight in the middle, but curving slightly 

 forward above, where it joins the hinge-line at an angle of 100° or 

 more, while below it curves more sharply into the rather narrowly 

 rounded base; anterior margin almost parallel with the posterior, 

 slightly convex in outline; surface with more or less distinct lines ot 

 growth, which are crossed on the anterior part of the valve by several 

 fine radiating lines and one or two stronger costae that extend above 

 the umbonal slope, giving it an angular appearance. These radiating 

 lines are usually distinct on young individuals and become obsolete or 

 disappear entirely on adult shells. Eight valve known only by frag- 

 mentary casts, which show that it was flat and smooth and did not 

 have a byssal notch. 



Height of left valve of a large specimen, 87 mm ; length of the hinge- 

 line, 52 ,nm ; convexity, 22 ram . 



The types of this species, which was originally described under the 

 two specific names cited above, came from the sandstones of the " first 

 ridge n at Coalville, Utah, where the fossils are preserved in the form 

 of casts that retain both the external and internal features to some ex- 

 tent without showing either very clearly. ■ Afterward Dr. White obtained 

 larger collections from the same place which enabled him to prove that 

 the two forms are not specifically distinct. In my collections made in 

 Huerfano park, Colorado, from about the same horizon there are several 

 better preserved specimens, clearly belonging to the same species, in 

 which the cartilage pits were observed, though the hinge can not be 

 well figured from the material in hand. The transverse folds along the 

 hinge-line, shown in one of the types, doubtless represent the cartilage 

 pits as modified by pressure, though, of course, they do not show their 

 form. Some of the casts show traces of two or three very oblique 

 linear teeth below the pits. Traces of the same hinge structure are 

 observable on some of the Coalville specimens. These facts are con- 

 sidered sufficient to justify the reference of the species to Gervillia. 

 The published figure of Gervillia gregaria Shumard, which comes from 

 approximately the same horizon in Texas, is very similar to small speci- 

 mens of this species. 



Locality and position. — Sandstones of the first and second ridges 

 about 800 feet above the lower bed of coal at Coalville, Utah. It also 

 occurs in the Pugnellus sandstone on Williams creek and Muddy creek, 

 Huerfano park, Colorado, and I have collected the same species near 

 the base of the Upper Cretaceous on the Chattahoochee river, 9 to 12 

 miles below Columbus, Ga. 



