Structure of Shell ik the Gents Orthis. 75 



species occasionally show a few large scattered pores or ducts near the 

 front of the shell. 



Professor King has proposed the name Platystrophia for Orthis 

 Mforata, and this species in its fibrous and non-punctate texture, may 

 be taken as characteristic of the first group, although there are some 

 features, especially in the form of the shell and also in the muscular 

 impressions, which do not in every respect agree with other members. 

 With our present knowledge, we may include in this group the fol- 

 lowing species : 



Platystrophia biforata, Trenton and Hudson Kiver groups. 



" tricenaria, " " " " 



" subquadrata, Hudson River group. 



" borealis, " " 



" occidentalis, ." " 



" plicatelia? " " 



" flabella,* Niagara group. 



" This list will be greatly extended as soon as the shell -structure of 

 allied species can be studied. At present only those species are included 

 which have been studied under the microscope. 



The second group embraces forms which are usually regarded as 

 typical species of the genus Orthis. The shell-structure characteriz- 

 ing this group may be described as finely fibrous, with distinct rows of 

 punctse coming out along the summit of the radii ; the rows of punctse 

 are simple, or double in some species (0. Clytie), but usually there are 

 several rows to each ray. 



The following species of this group have been microscopically studied 

 in numerous specimens: 



Orthis testudinaria, Trenton and Hudson River groups. 



" perveta, " " " " 



" Olytie, Hudson River group. 



" elegantula. Niagara group. 



" hybrida, ' " 



" Vanuxemi, Hamilton group. 



" Penelope, " " 



The third group, consisting of 0, multistriata of the Lower Helder- 

 berg group, 0. Iowensis of the Hamilton group, 0. Tulliensis of the 

 Tully Limestone, and 0. impressa of the Chemung group, is highly 

 punctate, with a fine fibrous texture of the shell-substance. In the 

 great number of the punctse and for the most part their uniform char- 

 acter, together with their arrangement, these forms of the Orthidce 

 resemble species of Terebratula, Cyrtina, etc. The name Schizopho- 

 ria, King, may be adopted for this latter group of species. 



The accompanying illustrations (plates 3 and 4) will serve to give a 

 clear idea of the microscopic characters presented in the shell-structure 

 of specimens in each of the three groups indicated. 



The preparation of the shell sections and the photographs of these 

 for the lithographer have been made by Mr. C. E. Beecher of the State 

 Museum. 



*Not Orthis flabella of Sowerlv. 



