79 



sial septum and dental plates of other genera; the dimensions of these varying almost in 

 every species ; they are most developed in Fentamerus Kni(//itif, vfhere the central septa 

 extend nearly to the frontal margin, while in other forms, such as in P. lens, the same 

 l)lates are small and rudimentary ; affording, as justly remarked by Prof McCoy, good 

 specific characters. The same proportions and differences arc likewise observable in the 

 arrangements of the smaller valve in Fentamerus 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 F. lena the dental plates extend freely into the cavity of 

 the shell, and that in ( F. liralus) they are produced in the form of free, long, and narrow 

 lamella^, to which the cirrated arms were of course attached. The exact position and form 

 of the different muscles has not been yet completely made out ; but in some .species the 

 quadruple impressions of the adductor are clearly defined." 



In Woodward's " Manual of the Mollusca," 1851-1856, p. 227, the 

 characters in question are thus alluded to : 



" Oral lamellae have been detected by Mr. Salter in P. liratus-. in P.? brevirostris (De- 

 vonian, Newton) the dorsal valve has a long trough-like process supported by a single low 

 septum." 



In the second edition of " Siluria,'' p. 229, (1859) Mr. Salter, to 

 whom Sir Roderick Murchison had confided the revision and notes of the 

 fossil data in his work, observes, while speaking of the Bi-achiopoda of 

 the Llandovery rocks, that : — 



" The Pentameri are, however, the characteristic fossils, which impart to this zone its 

 peculiar and distinct facies. No less than five species, whether smooth or only slightly 

 ribbed, occur, and of these P. ohlongus is the best known and the most widely spread. 

 This typical shell is easily distinguished from the other species, F. lens and F. liratus, by 

 the great length of the mesial septum, which in these latter is quite a short appendage to 

 the V shaped chamber. The two longitudinal plates, also, which divide the upper valve, 

 are peculiar to this species ; while in F. lens they are very short, and in F. liratus are 

 reduced to a pair of processes which pass inwards, but do not show upon the cast." 



This latter passage is quoted from Mr. Davidson's " British Fossil 

 Brachiopoda," vol. 3, p. 160, and he adds to it: — Lastly, Mr. Billings, 

 availing himself of the differenti. aracters pointed out by Mr. Salter, 

 proposed for such shells as P. liratus, P. lens, and some other Canadian 

 species, the generic designation of >S'^n(?A;/a?ic?da, and which he subsequently 

 altered to Strickland inia. This I will retain as a section of the large 

 genus Pentamerus, and in which I trust the species under description 

 will find a permanent home." ^ 



The genus was not founded altogether on the characters to which Mr. 

 Davidson alludes. While studying the species in the Canadian collection, 

 I observed that nearly all those with short plates in the dorsal valve 

 differed in general form from those with long plates. I, therefore, pro- 

 posed a new genus for their reception, and gave the following reasons for 

 so doing : 



Generic C haracters.—ShQU, usually large, elongate oval, transversely-oval, or circular 

 sometimes compressed; valves nearly equal; a short mesial septum in the interior of the 



