Cuming s — Morphogenesis of Platystrojpliia. 43 



The number of plications in the sinus varies from two to 

 nine (three to ten on the fold), which is also a greater range 

 than observed in any other type. All except the primary pli- 

 cations originate at a considerable distance from the beak. 

 The amount of variation in the shell index and contour (fig. 22) 

 is less than in the lynx type, and the sinus is never profound. 

 The average index is about 1*4, or very near that of the Tren- 

 ton lynx. The cardinal angle is difficult to determine with 

 accuracy owing to the worn condition of most of the speci- 

 mens ; but is scarcely ever less than 90° and may be consider- 

 ably over 100°. The dorsal valve is usually considerably 

 more convex than the ventral, especially in the umbonal 

 region, but the incurvature of the beaks is not as great as in 

 lynx. Internally hiforata has the dental sockets more deeply 

 excavated and the crura more prominent. The muscular scars 

 have not been observed. 



The British types of Platystrophia have given the writer 

 considerable trouble because of the general absence of speci- 

 mens of that province from American collections. The fig- 

 ures and descriptions given by Davidson and others throw 

 very little light upon the subject. All their Wenlock examples 

 certainly beloug to the dentata type, as is pointed out later, 

 but the position of the Ordovician forms cannot be deter- 

 mined. The var. fissicostata McCoy,* judging from the fig- 

 ures given by Davidsonf, strongly resembles our Clinton 

 forms. It has, according to McCoy, " four ribs on the ros- 

 tral part of the mesial furrow, the two outer of which usually 

 branch at four or five lines from the beak, the others branch 

 irregularly lower down once or twice ; lateral ribs varying 

 from six to fifteen within three or four lines from the beak, 

 branching irregularly, some into two, others into four, with 

 age ; surface crossed towards the margin with sharp striae of 

 growth. This variety does not seem to have been noticed 

 either in .Russia or America, yet it is extremely common in 

 our old rocks . . . . " This variety, according to Davidson, 

 " abounds in the Caradoc " at numerous localities and in the 

 Coniston, etc. Now, the only American type known to the 

 writer that ever has bifurcating plications is the hiforata of 

 the Clinton, in which group this peculiarity is common .% 

 Again, the presence of " four ribs on the rostral part of 

 the mesial furrow" is a character absolutely unknown in the 

 lynx type, but common in the hiforata type. On the whole, 

 it seems likely that fissicostata is a true hiforata similar to 



* British Pal. Foss., 1852, pp. 192, 193. 

 f Silurian Brachiopoda, 1871, pi. xxxviii, figs. 15-17, 19. 

 % Mr. Foerste has already called attention to this fact. Bull. Dennison 

 University, I, 1885, p. 80. 



