98 INTRODUCTION. 



acute, entire and much incurved, concealing a triangular fissure rarely exposed except in 

 the young state, and over which the umbo of the socket valve is greatly incurved ; no 

 area or deltidium ; exterior smooth, striated, or ribbed. Inside the larger valve, two 

 contiguous vertical septa coalesce into one median plate, extending from the beak to a 

 greater or less distance; and then diverge and form the dental plates, inclosing a 

 triangular chamber of much smaller dimensions than the lateral ones. 



In the interior of the smaller valve, instead of a single median plate there are two 

 distinct longitudinal septa of variable dimensions ; (between which a small median ridge 

 is occasionally found), to these the socket walls converge and join, forming two more 

 or less developed and inclined plates, to the produced extremities of which were affixed 

 the spiral cirrated arms. These plates often form a deep V shaped chamber corresponding 

 with the similar chamber in the opposite valve, and the edges of the two chambers are 

 applied against each other, so as to leave a rhomboid cavity between them. 



Ohs. The genus, Pentamerus was judiciously proposed by Sowerby ; ' but not always 

 well understood by subsequent authors. Dalman^ objected to Sowerby's name, on the 

 ground that the shell was not five chambered, and proposed as a substitute that of 

 Gypidia. Since then the minute and valuable researches of ]VI. de Verneuil/ Professor 

 King,* Barrande,^ M'Coy,® Suess, Salter, and others, have materially augmented our 

 knowledge of the internal dispositions and affinities which Pentamerus bears to other 

 genera. By the dispositions of the septa and dental plates in the ventral valve, it is 

 closely related to Camarophoria. The position of the mesial plate and V shaped process 

 in the dental valve has been clearly shown, both by Baron v. Buch and Professor King, to 

 be the equivalent of the mesial septum and dental plates of other genera ; the dimen- 

 sions of these varying almost in every species ; they are most developed in Pentamerus 

 Knightii, where the central septa extend nearly to the frontal margin, while in other forms, 

 such as P. lens, the same plates are small and rudimentary -^ affording, as justly 

 remarked by Professor M'Coy, good specific characters. The same proportions and 

 differences are likewise observable in the arrangements of the smaller valve ; in Pent. 

 Knightii, for example, the two sub-parallel longitudinal septa as well as the conjoined 

 and inclined dental plates, are much extended and elevated, while in other forms they 

 are considerably reduced, being almost rudimentary. Mr. Salter has observed that in 

 P. lens the dental plates extend freely into the cavity of the shell, and that in {P. liratus) 



' Min. Con., vol. i, 1813. 2 Petrefacta suecana, in 'Kongl. Vet. Acad. Handl.,' 1827. 



^ Geology of Russia and the Ural Mountains, vol. ii, 1845 ; and Count Keyserling's Wissenschaftliche 

 Beobachtungen, 1846. 



* Annals and Mag. of Nat. Hist., vol. xviii, 1846 ; and Mon. of Brit. Permian Fossils, 1849. 



^ Uber die Bracliiopoden der Silurischen Scbiehten, &c., 1847. 



6 Brit. Pal. Fossils in the Camb. Mus., 18.52. 



■^ See also the descriptions P. lens, oLlongus, undatus, microcamerus, by M'Coy, 'Brit. Pal. Fossils in 

 Camb. Mus.,' p. 209, &c. 



