ORTHIDyE. 277 



Ohs. Of this species 1 have seen two or three ventral valves only, and consequently 

 cannot offer a complete description of the shell. It was, however, identified as above by 

 Mr. Salter. Prof. Hall tells us that in the American type the dorsal valve is flat, 

 and regularly rounded from the extremities of the hinge-line; that its most striking 

 character, and one which will serve to distinguish it from any other Orthis known to him 

 in New York, is the great elevation of the ventral valve and the remarkably broad 

 cardinal area ; that the ribs are always simple, continuing distinct quite to the apex 

 of each valve, and gradually enlarging towards the base. Its interior is not known. 



Position and Locality. This shell was found by Mr. Salter and Mr. J. Thomson in 

 the Caradoc Limestone of Piedmont Glen, near Girvan, in Ayrshire, the specimens 

 being preserved in the Museum of Practical Geology. In ' Siluria ' it is stated to occur 

 both in the Caradoc and Llandovery (Lower, I suppose). In New York it is found in 

 the lower part of the Trenton Limestone, associated with Orthis testudinaria, Leptana 

 sericea, Atrypa triplex ; and at Middleville, and at Mineral Point (Wisconsin), apparently in 

 a higher part of the same rock. It occurs also in Canada. 



We have now described, very imperfectly it is true, all the British species of OrtJiis 

 with which we are acquainted. I am aware that several other so-termed species have 

 been proposed by different palaeontologists ; but as their descriptions and illustrations 

 are very incomplete and obscure, it would be useless to attempt to reproduce them in 

 the present state of our information. Among these we may mention Ortlds triangularis 

 (J. de C. Sowerby), the original drawing of which we have reproduced (PI. XXXVIII, 

 fig. 27). All that has seemingly been found of it is an incomplete cast of the interior 

 of one valve ; and this was obtained by Sir R. Murchison in the Caradoc Sandstone at 

 Marrington Dingle, near Chirbury. 



Orthis? productoides, M'Coy ('Synopsis Sil. Foss. of Ireland/ p. 32, pi. iii, fig. 15), 

 is another obscure fossil. I have seen the original example, which was kindly lent to me 

 by Sir R. Griffith ; but all the specimens are so incomplete and distorted that it is 

 impossible, on such material, to arrive at any correct or useful specific identification. 

 I have, however, reproduced Prof. M'Coy's illustration (PI. XXXVIII, figs. 29 and 30). 

 The fossil was obtained from the schists of Knockmahon, Tramore, County Waterford, 

 Ireland. 



Fragments also, which may, perhaps, eventually turn out good species, have 

 been collected by different geologists; but until more complete and satisfactory 

 specimens Avill have been found, it would be confusing the subject to attempt to describe 

 them in this Monograph ; nor can we admit, as well-determined Silurian species, a few of 

 those that are recorded in the ' Synopsis of the Silurian Fossils of Ireland.' 



