TRENTON FOSSILS. 295 



the margin, crossed by small elevated concentric lines. A prominent distinguishing trait of this shell is its 

 resupinate character, placing the area and foramen essentially on the dorsal valve, 

 which is nearly flat, while the ventral one is convex. 



Fig. 202 represents the ventral or convex valve of the Orthis pedinella, having radii 

 which are simple or nearly so. A specimen of this species in the State Cabinet is 

 from Isle La Motte, but without specification of the rock from which it was taken. 

 Another specimen of it was collected by us from the Black Itiver limestone at the island 



in Button Bay. 

 Orthis pectinella. * 



Orthis lynx (Hall.) Shell scarcely transverse, varying from semi-elliptical to 



subquadrate and globose, often nearly as thick as long; length and width as 5 to 9, or equal; cardinal 

 line often exceeding the width of the shell, and usually extended into short, acute ears, or rounded at the 

 extremities; area common to both valves, as also the foramen; ventral valve more gibbous than the 

 dorsal valve; surface marked with strong, angulated, longitudinal plaits, about three to four of which mark 

 the sinus, and four to five the elevated medial lobe; transversely ornamented by concentric or flexuous ele- 

 vated subimbricating lines, which are very obvious on the lower half of the shell, becoming very distinct in 

 front, and, in perfect specimens, continuing nearly to the beak. Examined with a magnifier, the surface is 

 covered with very fine granulations, like those in some specimens of recent Terebraiulce. 



It is impossible to assign any definite form and proportions to a shell as variable as this species. It is 

 greatly influenced by local circumstances, especially varying in different regions of the country. The species 

 in the eastern part of the American continent presents the following characters in its different stages of 

 growth and development. In the young shell, there first appears three plaits in the sinus of the dorsal 

 valve, with four on the corresponding medial lobe of the ventral valve; these four becoming two about one- 

 half or two-thirds of the distance from the base to the beak. As the shell grows older, another plait is 

 developed on one side of the sinus, and a corresponding one on the medial lobe ; but the five thus developed 

 become two before reaching the beak. In the larger specimens of the New York type there is a fifth plait 

 developed in the sinus, with six upon the corresponding medial lobe. The outer one on each side unites 

 with the adjoining one about half way to the beak, thus making four plaits on the medial lobe, which final- 

 ly unite in two before reaching the beak. The additional plaits of the medial lobe are developed laterally 

 by a division of the outer one, which takes place at nearly regular intervals corresponding to the increasing 

 size of the shell. The lateral plications in the sinus likewise disappear towards the beak, not by uniting 

 with the adjoining ones as on the opposite valve, but by gradual diminution till they are lost in the surface 

 of the shell. The plications on each side of the medial lobe and sinus increase in like manner by the devel- 

 opment of additional ones towards the margin of the shell; and from seven, the number usual in the 

 smallest shells, they increase to ten or eleven, the greatest observed number in any specimen presenting the 

 characters here given; the increase of lateral ones always keeping pace with the development of additional 

 ones in the sinus and medial lobe. 



Fig. 203 represents rather a large specimen of the Orthis lynx. It is found at McNeil's Point, Lar- 

 rabee's Point, north end of Isle La Motte, and probably at all important localities of Trenton limestone 

 fossils in Vermont. 



FIG. 203. 



SHELLS OF ACEPHALOUS MOLLUSKS. 



Tellinomya (Hall.) The shells of this genus are equivalve, inequilateral, 

 somewhat compressed below, but becoming gibbous at the umbones; umbones not 

 angular; outline of the shell curved, without angular ridges; shell thin, closely 

 laminated; hinge without visible teeth or crenulations; muscular impressions 

 two in each valve, near the dorsal margin, often apparently gaping at the poste- 

 rior extremity. 



Tellinomya levata (Hall.) Somewhat obliquely ovate or subrhomboidal, gibbous; anterior extremity 

 broad, rounded; posterior side narrower and somewhat obliquely truncated; umbones usually about one- 

 third of the length from the anterior extremity, elevated and incurved; cardinal line slightly curved; cren- 



