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- 

 o-uished 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 'purfctated 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 O. 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 favorably preserved oi- in 

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

 of this punctation, though it has, naturally, been impossible to study the shell-structure in every species 

 examined. It was originally a part of the plan and pui'pose of this work to take up the study of the minute 

 shell-stmcture in cormection with the genenc studies, and to include the result of the investig:ation in this 

 volume, as a part of the contribution to our knowledf^e of the PaliBozoic Brachiopoda. The lapse of nearly 

 twenty yeai'S fi'om the commencement of this work till it was ag-ain taken uji, in 1888,t has brought nnich 

 new material for c msideration, while the studies and publications of numei-ous authors have served to 

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

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

 foi- the present, but it is the idtimate purpose of the author to take up as a special subject the examination 

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



t The work ol' a revision of the genera of the Palaeozoic Brachiopoda was began soon after the completion and pub- 

 lication of Vohime IV of the Pal.Tontology of Xew York, and thirty plates, beginning witli ilhistrations of the generic 

 group of Oiniiis had already hcou lithographed, from drawings chicny made by I\[r. R. P. WurrriELD, when it hcranie 

 necessary to suspend the work. These plates, originally numl)ered from aV to XXXVI, were mostly lithographed by Mr. 

 PHli.ir AST; two of them by Swinton, two by Riemann and two by Bkhgman, between tlie years 1S70 and 1876. Of 

 these plates aV and V have been cancelled, and, witli large additional material and accumulated infonnation, have been 

 substituted by V, Va, Vb, Vo. The original plates, from VI In XIX. are included in this volume, and are supplemented by 

 VU, VUa, XIa, XIb, XIC, XlD, XVa, XX. 



