338 Thirty-fifth Eeport on the State Museum. 



vex, gibbous in the unibonal region. Eight valve smaller and less 

 convex. Hinge-line straight, longer than the length of the shell. 

 Beaks acute, situated near the anterior extremity of the hinge, 

 prominent, inclined forward, arching over the hinge-line. Umbonal 

 region ample, subtending a right angle. Ear small, bending down- 

 ward, denned by a broad, shallow sulcus. Byssal sinus narrow. 

 Wing large, triangular, greatly extended along the hiuge-line, de- 

 fined by the retral bending of the striae ; margin concave ; ex- 

 tremity acute. The wing of the right valve is very broad, flat, and 

 much extended ; not distinctly defined. 



Surface marked by fine, closely arranged striae of growth, with 

 distant, elevated lamellae, which are extensions of strong fascicles 

 of striae. These leave angular concentric folds and undulations in 

 the casts. The striae are strongly marked on the cardinal expan- 

 sions and over the hinge-margin. In well-preserved specimens the 

 undulations are stronger upon the middle of the valve, and less 

 marked upon the anterior side and the wing. In the right valve 

 they are much more regular and subdued. Interior characters 

 unknown. Ligamental area marked by a distinct groove parallel 

 to the margin. 



The largest specimen has a length of 50 mm., height 46 mm., 

 hinge-line about 45 mm. A small specimen measures 26 mm. in 

 length, 39 mm. in height, hinge-line imperfect. Another specimen 

 is 26 mm. in length, 33 mm. in height, hinge-line 27 mm. A right 

 valve is 33 mm. in length, height about the same, hinge-line 41 

 mm. 



This form resembles in general surface characters, the three pre- 

 ceding species, but differs by the more erect body and sub-orbicular 

 form. In general aspect it resembles G-lyptodesma, and the surface 

 markings are not very different, though none of the specimens show 

 the hinge characters of that genus. The description has been drawn 

 from specimens preserved in fine calcareous shale and in coarser 

 arenaceous material, and the characters remain essentially the same 

 in both. 



In a single specimen from the arenaceous shales on the shore of 

 Cayuga lake, at Norton's Landing, where the shell is partially ex- 

 foliated, the surface is marked by fine, concentric lines of growth, 

 and more distant lamellose striae, which are about twice as numerous 

 as in the typical specimens. These give a gently undulating character 

 to the surface of the cast of the left valve, instead of the abrupt 

 angular folds of the typical specimens. In the right valve of the 

 same specimen the striae on the umbo are similar, but more closely 



