ORTHID^. 269 



Spec. Char. Transversely semi-elliptical or subquadrate, wider than long, more or less 

 globose, the length, width, and depth varying sometimes but little ; liinge-line more often 

 rather less than the width of the shell, sometimes slightly exceeding the general breadth, 

 with short acute mucronate wings, or rounded terminations ; in front the ventral valve is 

 abruptly deflected and indents the opposite one ; beaks in both valves much incurved and 

 approximating. Ventral valve convex, with a wide deep medial sinus commencing at the 

 extremity of the beak and widening as it nears the front. Area triangular, of moderate 

 height, fissure open, beak angular, incurved Dorsal valve deeper than the opposite one, 

 at times gibbous, with a wide longitudinal fold, commencing at the extremity of the ura- 

 bonal beak and extending to the front ; area a little less wide than in the opposite valve, 

 fissure open. Surface of both valves ornamented with a greater or lesser number of 

 radiating angular ribs ; of these from one to five (and in some varieties more) furrow 

 the medio-longitudinal sinus, while from two to six or seven compose the mesial fold. 

 The valves are also crossed at intervals by numerous concentric, raised, subimbricating 

 lines ; the surface is also marked with small punctures. In the interior of the ventral 

 valve a prominent hinge-tooth exists on each side of the fissure and is supported 

 by strong dental plates, which enclose an elongated, oval, raised muscular cavity of 

 moderate dimensions. In the interior of the dorsal valve no prominent cardinal process 

 is observable, but two short brachial processes deviate from the extremity of the umbonal 

 beak, and on the outer side of these are situate the hinge-sockets. The quadruple muscle 

 forms four very distinct cavities, strongly margined and divided longitudinally and trans- 

 versely by prominent cross-like ridges. 



Length II, width 13, depth 9 lines. 



Obs. This important Silurian type has been often well described and illustrated, but 

 palaeontologists have differed considerably (as may be seen from a glance at the long hst of 

 synonyms) with reference both to the genus and the specific denomination the shell should 

 retain. It has consequently been assigned to no less than six difi'erent genera and to ten 

 or more species or named varieties. As to the genus, I think I was able to satisfactorily 

 demonstrate in 1848 that its internal characters were those of Orthis, and I am glad to find 

 that palaeontologists very generally have followed ray identification. Prof. King, in 

 1849, proposed to consider it the type (^f a new genus, Platystrophia^ but no other 

 palaeontologist that I am aware of has adopted that view. For many years it was regarded 

 and described as a Spirifer ; but a glance at its interior appears to me sufticient 

 to show that it cannot belong to that genus, any more than to Terebratula, Atrypa, or 

 Porambonites. It is also an exceedingly variable shell, and as it occurs abundantly in 

 America, and as it is there that it has attained its largest dimensions, I will 

 append an extract from p. 133 of the first volume of the 'Palaeontology of New York,' 

 in which Prof. Hall clearly explains the cause of its extraordinary variability : — " It is 

 impossible to assign any definite form or proportions to a shell as variable as this species. 

 The cardinal line, in young shells, usually terminates in small acute ears extending beyond 



