218 PALEONTOLOGY OF NEW YORK. 



together with 0. platys, Billings, from the Chazy, 0. Saffordi, sp. nov., and 

 probably 0. Holstoni, SafFord, from the Trenton, form a small group distin- 

 guished by its peculiar exterior, though the internal characters of the species 

 are still undetermined. To establish the true generic value of all these forms 

 will require much patient work both in the field and in the laboratory. 



The orthids occurring in primordial faunas have in so many instances shown 

 a comprehensive structure, having characters which individually are distinctive 

 of Orthis, Orthothetes, Clitambonites, Scenidium, etc., that it may be ques.- 

 tioned whether any of these primordial forms can be included under Orthis 

 according to the strict definition of the term, or even under any of the sub- 

 divisions here proposed. 



The development of the punctated shell-structure in this genus is a peculiar 

 phenomenon.* In eight of the thirteen proposed subdivisions of Orthis, the 

 shell-structure is prismatic but impunctate. So far as now known there is not 

 an impunctate Orthis in the faunas later than the Silurian. On the other hand 

 the punctate species are decidedly in the minority in Silurian faunas, attaining 

 their great numerical development in the Devonian. The first appearance of 

 punctation is along the line of the 0. testudinaria group (Dalmanella), but evi- 

 dence is still required to show that some of the earliest species included in this 



* Thin sections of these shells have been made whenever material has been favoi'ably jii'eserved oi- in 

 sufficient quantity to allow it, and no evidence has been found of an indiscriminate or sporadic appearance 

 of this punctation, thoug-h it has, natui-ally, been impossible to study the shell-structure in e\-ery species 

 examined. It was oi'ig-inally a part of the plan and purpose of this work to take up the stuily of the minute 

 shell-structui'e in connection with the generic studies, and to include the result of the investigation in this 

 volume, as a part of the contribution to our knowledge of the Palaeozoic Brachiopoda. The lapse of nearly 

 twenty years fi-om the commencement of this work till it was again taken up, in 1888,t has brought much 

 new material for cinsidei'ation, while the studies aiid publications of numerous authors have served to 

 present the subject in new aspects, and at the same time to demand a somewhat ditfei'ent ti-eatment from 

 that originally contemplated. Owing to these conditions the study of the shell-structure has been postponed 

 for the present, but it is the ultimate purpose of the authoi' to take up as a special subject the examination 

 of the minute shell-structure in the different genera of Palaeozoic Brachiopoda. 



t The work of a revision of the genera of the Palaeozoic Brachiopoda was begun soon after the completion and pub- 

 lication of Volume IV of the PaU-eontology of New Yorli, and thirty plates, beginning with illustrations of the generic 

 group of Orthis had already been lithographed, from drawings chiefly made by Mr. R. P. Whitfield, when it became 

 necessary to suspend tUe work. These plates, originally numbered from aV to XXXVI, were mostly lithographed by Mr. 

 PiIILlF AST; two of them by SwiNTON, two by R.iemann and two by Bekgman, between the years 1S70 and 1876. Of 

 these plates aV and V have been cancelled, and, with large additional material and accumulated information, have been 

 substituted by V, Va, Vb, Vc. The original plates, from VI to SIX. are included in this volume, and are supplemented by 

 VIA, VIlA, XIA, XlE, XIC, XlD, XVa, XX. 



