34 INTRODUCTION. 



upon its external surface in rows along the ridges (Plate V, fig. 10). They are also very 

 evident in the fossil Th. leptcenoides ; but I am not by any means sure of their presence in 

 Tli.mayalis, Moorei, sinuata, and triangularis. In the specimens of these which I have 

 examined, however, the shell was so far from being well preserved, that I do not feel 

 inclined to speak confidently upon the point. 



Family— SPIRIFERIDtE. 



In all the genera of this family, the structure of the shell is completely conformable to 

 the general description already given ; but the system of shell-canals no longer exhibits 

 the same constancy, being, in fact, more generally absent than present. 



Spirifer. — In this genus, as usually constituted, there is a great diversity in regard 

 to the presence or absence of the perforations ; but Mr. Davidson has been led, by con- 

 siderations altogether independent of them, to divide it in such a manner that the per- 

 forated and non-perforated species fall into distinct sections. Thus, he makes his genus 

 /S);/r//(?r-proper to consist of those species which, like 8p. striatus, are unpunctuated, and 

 without a mesial septum in the larger or ventral valve. Under this head rank >S^. com- 

 primatus, concentricus, speciosus, and Verneuilli, which I can state with confidence to be 

 non-perforated (notwithstanding Professor King's assertion to the contrary), not the least 

 trace of passages being discernible in sections which display the texture of the shell in the 

 greatest perfection. Under the sub-genus Cyrthia, he places these unpunctuated species, 

 which, like C. trapezoidalis, have a perforation for the passage of a pedicle ; besides this 

 species, I have examined C. cuspidata, and am fully satisfied that in neither of these do 

 any perforations exist. Under the sub-genus Spiriferina, on the other hand, he ranks all 

 the species which, like Sp. rostrata, are punctuated, and which have a largely-developed 

 mesial septum in the ventral valve. I have determined the existence of perforations in 

 Sp. Demarlii, heteroclyta, rostrata} and Walcotii. 



Athyris. — In this genus, also, it is proposed by Mr. Davidson to separate the perforated 

 and non-perforated species, which fall into different sub-generic sections respectively, 

 according to other features in their structure. The typical species of the genus, such as 

 A. herculea and A. tiimida, I have ascertained to be non-perforated. The same is the 

 case with concentrica, which Mr. Davidson now ranks under the sectional type Spirigera, 

 with other unpunctuated species. On the other hand, the punctuated species, such as 

 esquerra and melonica, in which I have found the perforations to exist, are placed by him 

 under the sectional type Retzia; and he thinks it probable that Ter. Salteri will have to be 

 transferred to this sub-genus. 



Uncites. — Of this genus, I have examined only U. gryphus, which is not perforated. 



Atrypa. — In none of the species of this genus which I have examined, namely 

 A. ajjinis, marginalls, and reticularis, is there the least trace of perforations. 



' PI. V, fi-. 3. 



