Tropidoleptus Fauna at Canandaicua Lakk, N. \'. 107 



Changes During Development. 



In the earliest stage in which striae appear there are from eleven to 

 fifteen. New ones are added by intercalation at an early stage so that 

 the smallest specimen, 1.4 mm. long, has 18 striae. In the adult 

 there are from 24 to 30. In all other characters than the strine the 

 development is the same as in O. chemiingensis. The index is very 

 constant, for, neglecting the nepionic stage where it is practically 

 I, it varies from 1.25 to 1.50. 



Specific Characters of the Shell. 



As Developed in the Upper Haniilton. — Biconvex or plano-convex, 

 transversely elongate, with a hinge width less than the greatest width 

 below. Area of ventral value high, frequently distorted. Dorsal 

 area linear. Delthyrium completely closed by a very convex deltid- 

 ium, which, in young stages, is prolonged into an exert pedicle tube. 

 In the adult the pedicle passage is closed. 



Surface marked by 24 to 30 strong simple plications, which increase 

 by implantation. Between them are spaces which are wider than the 

 plications. Very numerous concentric lines cover the entire surface. 

 Shell substance highly punctate. The punctge lie in double rows 

 between the plications. 



A species, which, if not identical with this, is at least very closely 

 related, is Orthothetes flabellus White, from the Upper Helderberg of 

 Columbus, Ohio. This species is larger than the specimens in the 

 present collection and has the greatest width at the hinge. The striae, 

 which are simple, number 22 or 24 and are separated by wide concave 

 interspaces. The specimen figured is 22 mm. long and 26 mm. in 

 width. ^ 



Orthothetes : Summary. — The development of two other species 

 of Orthothetes has been worked out. That of O. subplanus of the 

 Niagara by Beecher and Clarke '' and of O. minutus of the Salem 

 Limestone by Cumings.^ 



O. bellulus has in its adult stage many characters which agree with 

 the neanic stages of O. minutus. That species has in its earliest pli- 

 cated stage eighteen plications and more are added in the same way 



' See Annals N. Y. Acad. Sd., 1891, p. 521, pi. 6, figs. 7-9. 

 ''■Memoirs N. V. State Museum, Vol. I, No. I, 1887. 



3 Orthotlietes minutus, Am. Geol., Vol. XXVII, March, 1901, p. 147, pi. XVF, 

 figs. I-II. 



