10() AXXALS OK THK CaRNEGIE MuSEUM. 



The Same Species in Other Localities. 



The specimens from Pompey Hill, N. Y., are all small. The 

 largest is 17x25 mm. and the smallest 8X12 mm. A specimen 

 13 X 19.5 mm., which is of average size for this locality, has 100 

 stri^. The dorsal valves of these specimens are nearly flat, and the 

 ventral valves low, but distorted about the umbo. 



The specimens from Pratt's Falls, N. Y., are of about the same size 

 and proportions. The smallest is 5 X 7 mm. Another is 8.5 X 

 12.5 mm. and has 70 stride. A third is 13 x 20 mm. with no striae 

 and the largest specimen is 17.5 x 23 mm. These specimens have 

 very low cardinal areas and are hardly at all distorted. 



The specimens from both localities belong to the variety arctistriatiis. 



Ortothetes bellulus Clarke. (Plate V, Row 3.) 



Clarke, 13 An. Report N. Y. .State Geol., 1895, pp. 176, 187, pi. 4, figs. 2-4. 



This species is represented by a less complete series than O. che- 

 mungensis. The range in size is from 1.4 X 2.3 mm. to 10 X 14.5mm. 

 Between these limits there are specimens which show all stages up to 

 the adult condition, but none show senile characters and probably the 

 normal size of the adult is not attained, judging from the case of the 

 other species of this genus. 



The individuals figured by Clarke differ in minor respects from the 

 present specimens and a portion of his description of the Marcellus 

 species is quoted. " This species is of persistently small size and quite 

 regular in form, none of the specimens showing umbonal distortion. 

 The valves are transversely elongate, the hinge line forming the 

 greatest diameter. The surface is covered with eighteen or twenty 

 sharp, narrow, distant plications, . . . -and in the middle of each of 

 the broad interspaces is a smaller and usually obscure rib : rarely two 

 of these ribs are present. The length of an average specimen is 

 10 mm.: its diameter along the hinge is 15 mm. This is from the 

 limestone of the Marcellus division at 854 feet in the (Livonia Salt) 

 shaft." 



Nepionic Stage. — The shell is very small in the nepionic stage. It 

 is almost circular, biconvex and smooth. The diameter is from .10 

 to . 1 2 mm. as preserved on the beaks of dorsal valves from 1.4 to 

 3 mm. in length. 



