Tropidoi.eptus Fauna at Canandaigua Lakf., N. Y. S7 



the posterior end of the shield is a narrow, crescent-shaped, transverse 

 scar (Fig. 2). 



In the dorsal valve the arrangement is much the same, except that 

 the anterior pair of scars, instead of being oval and oblique, are 

 elongate and run back to the i)Osterior transverse scar. The scars of 

 the posterior pair are situated outside of the narrow part of the ante- 

 rior scars, and are slightly smaller than the corresponding scars in the 

 opposite valve (Fig. 3). 



The main scars probably are analogous to the anterior and posterior 

 occulsors of Crania, and the posterior scar may have the same func- 

 tions as the umbonal scar in Lingula. A very similar arrangement ot 

 scars is seen in Treinafis (Fig. 4). 



Stropheodonta inaequistriata Conrad. (Plate 111, Rows 3, 4, 5.) 



Conrad, Pal. N. V., 1S67, p. 93, pi. 12, figs. 6-8. 



In the Canandaigua Lake material there are about four hundred 

 well preserved pedicle valves, and about one-half as many dorsal valves 

 of this species. The dorsal valves are not so well preser\ed as the 

 ventral valves, as the anterior part is usually broken away. Only 

 about twenty specimens which retain both valves were obtained, and 

 these were nearly all adults. 



The smallest specimen in the collection is 1.2 mm. long and 2.2 

 mm. wide. The largest is 16 mm. long and 27 mm. wide. All grada- 

 tions between the two are shown. Hall (Pal. N. Y., Yo\. IV, plates 

 15 and 18) figures several specimens, all adult or senile individuals 

 form the Corniferous and Hamilton. The smallest is 12 x 18 mm. 

 and the largest 22 x 29 mm. The specimen figured on Plate 18, fig. 

 2X', Pal. N. Y., Vol. 4, shows the muscle scars in the ventral valve of 

 a senile individual, and fig. 10, pi. 15, shows the muscle scars of the 

 adult. Fig. 1 1 on the same plate shows well the interior of the dor- 

 sal valve, with the adductor scars (which are not represented as 

 divided into two pairs) and the strongly raised curved ridges in front 

 of the scars of the senile individual. Fig. 2h, pi. 18, shows the mus- 

 cle scars and ridges of a younger specimen, probably in the early 

 ephebic stage. The species is placed by Hall and Clarke with S. 

 arcitata Hall, S. cayuta Hall and S. variabilis Calvin, in the division 

 Douvillina of CEhlert. 



Nepiotiic Stage. — In the nepionic stage the shell is oval in outline 

 and wider than long. Both valves are convex, though the dorsal 

 valve becomes fiat in front in some specimens. In the dorsal valve 



