ANIMALS. 123 



Strigoceplialus, the validity of Mr. Gray's fundamental classification, which I have 

 adopted, would have been materially prejudiced. Whether the junction of the spirals, 

 by means of their crura, is a character general to the present family, or peculiar to one 

 or more of its subdivisions, I am not prepared to say ; I entertain an idea, however, 

 that it will be found to prevail to a much larger extent than we are at present 

 aware of.^ 



Another important character of the spirals elucidated through the researches of 

 Mr. Davidson, is the spine-like processes with which they are armed in certain 

 Jurassic species.2 Something similar has evidently characterised the spirals of 

 Cleiothyris pedinifera ; as all the examples I have seen of these appendages, in this 

 species, are pectinated throughout their entire length, as exhibited in fig. 10, PI. X. 

 Mr. Davidson states, that Professor Owen " thinks they are calcareous excrescences 

 destined, perhaps, to support the cilia."^ This is extremely probable, considering how 

 very elongated the cilia (^ cirri, Forbes) are in Terebratida caput- serp eniis ;^ but 

 without entertaining an opinion adverse to the view just noticed, I cannot but make the 

 suggestion, from the rigid and enduring nature of the brachial cirri in certain Terehra- 

 tula, that the spine-like processes on the spirals may be the cirri themselves fossilized. 



As regard the dental and other internal plates of SpiriferidcB, there evidently 

 prevail some important modifications. Professor M'Coy,^ Mr. Davidson's,® and my own 

 observations have made known, that between the dental plates of certain Spiriferidse 

 there is an additional plate,^ which I feel confident is absent or considerably modified 

 in several others, as Trigonotreta alata and T. undulata. I have elsewhere termed this 

 structure the ventral median plate, and considered it to be a muscular fulcrum (vide ante, 

 pp. 68, 70). The dental plates appear never to be absent, though they exist under 

 various degrees of development in different species : in Trigonotreta Mosquensis they are 

 large and very much elongated, almost reaching to the front of the shell ; but in 

 T. alata they are little more than rudimentary ; whereas in T. cgrtana and T. rostrata, 

 Zeiten, they are intermediate in size : they are generally attached to the inside of the 

 valve ; but in Spirifer heteroclytus they are cemented to the lower part of the sides of 

 the median plate, forming an acute arch-shaped process similar to that of Camarophoria 



1 Mr. J. de C. Sowerby's figure of the spirals of Cleiothyris pectinif era (Min, Conch., pi. 616) represents 

 the crural processes closely approximated : perhaps they are conjoined ? I regret that none of my specimens 

 throw any light on this important question. 



2 Vide London Geological Journal, vol. i, p. 111. 

 ^ Loc. cit. 



* Vide Forbes and Hanley's British Mollusca, vol. ii, p. 355, pi. U, fig. 1. 



^ Vide Synopsis of the Characters of the Carboniferous Fossils of Ireland, p. 127, fig- 14 c, 1844. 



6 Vide London Geological Journal, pi. xviii figs. 2, 4. 



7 Vide Ann. and Mag. of Nat. Hist., vol. xviii, p. 86, 1846. "In Spirifer cristatus, S. Walcotti, 

 S. rostratus, Zeiten, Martinia imbricata, &c., this plate, -which is large, is situated between, and independent 

 of, the condyle plates." (Loc. cit.) 



