ANIMALS. 123 
Strigocephalus, the validity of Mr. Gray’s fundamental classification, wmch 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, Iam 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. Something similar has evidently characterised the spirals of 
Cleiothyris pectinifera; as all the examples I have seen of these appendages, in this 
species, are pectinated throughout their entire length, as exhibited in fig. 10, Pl. 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 Zerebratula caput-serpentis ;* 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 7erebra- 
tule, that the spine-like processes on the spirals may be the cirri themselves fossilized. 
As regard the dental and other internal plates of Spiriferide, 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 Spiriferide 
there is an additional plate,’ which I feel confident is absent or considerably modified 
in several others, as Trigonotreta alata and T. undulata. 1 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 Zrigonotreta Mosquensis they are 
large and very much elongated, almost reaching to the front of the shell; but in 
Tf. alata they are little more than rudimentary ; whereas in 7. cyrf@na 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 Cletothyris pectinifera (Min. Conch., pl. 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. 
3 Loe. cit. 
4 Vide Forbes and Hanley’s British Mollusca, vol. ii, p. 355, pl. U, fig. 1. 
5 Vide Synopsis of the Characters of the Carboniferous Fossils of Ireland, p. 127, fig. 14, 1844. 
6 Vide London Geological Journal, pl. 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.”’ (Loe. cit.) 
