288 Raymond — Common Devonian Brachiopods. 



Lake locality differ only slightly from the type. As described 

 by Clarke, the Marcellus specimens have from 18 to 20 plica- 

 tions; the greatest diameter is at the hinge, and there is no 

 umbonal distortion. The Hamilton specimens, however, have 

 from 24 to 30 plications, the width at the hinge is less than that 

 below, and the cardinal area is high, the ventral umbo being 

 on that account frequently distorted. 



The developmental stages offer nothing new. The shell 

 in the nepionic stage is very small, usually from *10 to •12 mm in 

 length. In young stages there is an exsert pedicle tube, while 

 in the adult a strong convex deltidium covers the delthyrium. 



Summary. — The development of two other species of Ortho- 

 thetes has been worked out : That of 0. subplanum, of the 

 Niagara, by Beecher and Clarke,* and that of 0. minutus of 

 the Salem limestone, by Cumings.f 0. hellulus has in its 

 adult stage many characters which agree with the neanic stages 

 of O. minutus. The latter species has, in its earliest plicated 

 stage, 18 plications, and more are added in the same way as in 

 0. hellulus. In the adult, dwarfed, there are 40 plications. . 



Comparing the development of 0. suhplanum of the Niagara 

 with that of 0. chemungensis, many differences are found. 



First: Convexity of Valves. — The nepionic shells of 0. 

 chemungensis have the valves subequally convex, which is the 

 adult state of 0. subplanum, while its nepionic shell is concavo- 

 convex. 



Second.— In 0. subploMum the cardinal area is low compared 

 with the length of the hinge line and the adults are symmetri- 

 cal. In 0. chemungensis only in the early neanic stages are 

 the shells perfectly symmetrical usually, though some speci- 

 mens retain the low cardinal area and symmetry into the late 

 neanic stages. 



Third : Surface Characters. — The duration of the nepionic 

 stage is about the same in both species. In the youngest 

 specimen of 0. subplanum figured by Beecher and Clarke, 

 2-25 mm long, there are IT striae, 6 of which are secondary. A 

 specimen of O. chemungensis of similar length has from 28 to 

 35 striae. 



Fourth : Hinge Structure. — The young of both species have 

 a strong convex deltidium which is prolonged into an exsert 

 tube for the passage of the pedicle. In 0. subplanum the 

 deltidium ceases to grow at an early stage, while in 0. chemung- 

 ensis the deltidium continues to increase in size throughout 

 all the stages. 



* Silurian Brachiopoda. Memoirs N. Y. State Museum, vol. i, No. 1, 1889, 

 p. 23, pi. ii, figs. 14-20. 



f Am. Geol., vol. xxvii, March, 1901, p. 147, pi. xv, figs. 1-11. 



