74 Thirty-sixth Report ok the State Museum. 



ventral. There are also resupinate forms which are closely allied to 

 them; but, as a rule, the forms with finely striated surface, subequi- 

 valve or plano-convex, have the ventral valve the more convex ; and 

 the muscular impression is flabelliform with its margins lobed, and 

 more or less distinctly limited by an elevation of the interior substance 

 of the shell. 



The resupinate forms which are more closely allied to those with 

 flabelliform ventral muscular impressions, have the corresponding mus- 

 cular imprint more strongly defined and less distinctly lobed at the 

 margins than in the forms just noticed. 



These are the most obvious distinctions among the prevailing forms 

 of the genus Orthis as constituted by Dalman. 



The Orthis (Platystrophia biforata) is, in some degree, an excep- 

 tion to all the forms above mentioned, having both valves very convex, 

 the surface strongly plicated, with a mesial fold and sinus, as in 

 Spirlfera. In its muscular areas it resembles the resupinate forms of 

 Orthis of the lower Silurian rocks, often presenting an abnormal thick- 

 ening of the shell around the muscular area of the ventral valve. 



The Orthis tiloba {Diccelosia bilola of King) of the upper Silurian 

 rock also presents a departure from the typical forms of Orthis, but 

 preserves the similar muscular system. 



Before undertaking a revision of the materials constituting the 

 genus Orthis, it has seemed desirable to ascertain whether the varia- 

 tions in form, surface ornamentation, or character of muscular impres- 

 sion, is associated with any difference in the shell-structure. For this 

 purpose, cuttings, prepared for microscopic examination, have been 

 made from many species, and the result has proved that all the resu- 

 pinate lower and middle Silurian forms, whether plicate or finely stri- 

 ate, are fibrous shells, with the ventral muscular impression small 

 and strongly limited. They are essentially either free from punctas in 

 any form, or with a few scattered pustuliform pores. The finely stri- 

 ate sub-equivalve or plano-convex forms with flabelliform muscular im- 

 pressions, have the shell punctate in lines, or radiating belts, corre- 

 sponding to the rays of the shell, with an intermediate fibrous texture. 

 The character of the punctse, the strength and comparative width of 

 the punctate bands, vary with the different species. 



In the finely striated, resupinate forms of the Lower Helderberg, 

 Hamilton and Chemung groups, with the smaller and more distinctly 

 limited flabelliform ventral muscular areas, the punctate character is 

 very marked, often occupying almost the entire surface, and the lines 

 of the radii are shown only by a more crowded condition of the punctae. 

 The resurnnate species here referred to are quite different in their out- 

 line and general form from those of the lower rocks, being for the 

 most part rotund forms with the cardinal extremities rounded. These 

 species are easily recognized, and readily distinguished from those of 

 the preceding group by their external form alone. 



The numerous species which have been already studied in their 

 microscopic shell-structure are naturally separated into three distinct 

 groups which may be of generic value. 



The first group includes the coarsely plicate forms, with extended 

 cardinal angles ; the valves resupinate or normal in their relations ; 

 the test is coarsely fibrous, and usually without punctse, although some 



