100 INTRODUCTION. 



mesial elongated triangular fissure, which truncates the beak of the larger valve before ex- 

 tending to the hinge line ; surface pitted, but the shell structure impunctate ;^ in the 

 interior of larger or ventral valve the dental plates form two slightly elevated and diverging 

 septa, extending to a greater or less distance along the bottom of the shell. In the other 

 valve the socket plates form likewise diverging septa, extending also to a variable distance. 



Ohs. The genus Porambonites was proposed by Pander for a remarkable group of 

 shells which abound in the neighbourhood of St. Petersburgh, but from the difficulty 

 of obtaining interiors, they have been differently interpreted by several authors. 

 M. de Verneuil seems uncertain as to the place these shells should occupy, since he states 

 that he considers them to be nearer related to Spirifer than Terebratula, although removed 

 by various characters, and principally by the septa of the socket valve, so that they might 

 equally well be classed with Orthis. Since then M. d'Orbigny^ has replaced the shells in 

 Pander's genus, and admitted them into his family of Rhpichonellida, from a belief that 

 the free spiral fleshy arms were probably fixed to the socket plates. Prom the beaks 

 being in general so much incurved as almost to come in contact, the extent of the 

 foramen or fissure is concealed, so that some authors, as for instance M. d'Orbigny 

 and Professor King,^ state only one of the beaks to be foraminated, while, on the contrary, 

 M. de Verneuil,* Professor M'Coy,^ Dr. Volborth/ and Professor Quenstedt,'' believe 

 that both possessed a foramen or fissure. Some additional light on the internal 

 arrangements has been obtained through Mr. Sharpe's fortunate discovery of several 

 internal casts of a Portuguese species {P . Ribeiro) ; we have figured, with that gentleman's 

 permission, in PI. VII, figs. 225-26, one which exhibits, in a beautiful manner, not 

 only the dental and socket plates, but the position of the muscles and vascular impres- 

 sions, not hitherto observed in any of the other forms ; but the internal details of the 

 different Russian species, require still further examination before the exact character of 

 the genus wiU be completely known. 



Geol. range. — The genus seems to be confined to the lower Silurian period. 



Examples ■■ P. aquirostris, Sch., sp. ; intercedens^ Pander ; reticulatus, V. Buch, sp. ; 



Bibeiro, Sharpe, &c. 



1 The depressions or dots visible on the surface of the different species were found by Dr. Carpenter 

 to be simply due to external sculpture, the shell structure being impunctate. 



* Considerations Zool. et Geol. sur les Brach., in the ' Comptes rendus Heb. de I'Acad. des Sciences of 

 Paris,' 1847 ; and Pal6ont. Franc. Ter. Cretaces, vol. iv. 



' A Monograph of English Permian Fossils, p. 112, 1849. 



* Geol. of Russia and the Ural Mountains, vol. ii, 1845. 



'" British Pal. Fossils in the Camb. Museum, p. 212, 1852. 



^ This gentleman has informed me, " that the area is in general so small as hardly to deserve the 

 name, but that the triangular fissure is large and the teeth developed." 



'^ Handbnch der Petref., p. 486, 1851. The author states that both beaks are perforated at their 

 extremities : he likewise illustrates a fragment of the interior of one of the species, in pi. xxxix, fig. 5. 



* Pander describes many species which have turned out to be mere varieties and synonymes. It is 

 likewise doubtful, if the Sp. Tcheffkini of De Verneuil belongs to this genus. 



