144 SYNOPSIS OF THE CHARACTERS OF THE 



Reticularia striatella. M'Coy. (PL XIX. fig. 13). 



Sp. Ch. — Transversely oval, or, including the beak, subrhomboidal, gibbous ; front very slightly elevated ; 

 surface radiated with numerous, fine, close, rounded striae. 



The specimen figured of this pretty little shell shews the remarkable parallelism of the dental lamlnaj so 

 characteristic of the subgenus Reticulai-ia, M'Coy, it also has the same general form, small size, and absence of 

 a distinct mesial fold, as in the other species of that very definite little group ; the longitudinal strias are very 

 numerous, fine, and equal. Length six lines, width eight lines. 



Brachythyris. il/' Coy. 



Gen. Ch. — Hinge-line shorter than the width of the shell; cardinal area small, curved, triangular ; surface 

 ribbed longitudinally. 



The species which I bring together under this name, form a very remarkable little group, possessing the 

 short hinge-line, and triangular area of the genus Martinla, M'Coy, with the ribbed surface of tlie true Spirifers. 

 Holding characters which are essentially intermediate between the more positive distinctions of the more 

 typical groups, it could scarcely be supposed, that the characters of the genus should be so constant as they 

 appear to be. Professor Phillips has observed the strong affinity which exists among the species, by making 

 of them his third division Kadiatce, of his great group Spirifera. Some of this genus, as Spirifera exarata, 

 Fleming, and Brachythyris planicostata, M'Coy, possess the singular peculiarity of their large valve being found 

 in great quantities separated from the smaller ; from which I would infer, that the cardinal teeth of this genus 

 must be either much smaller or weaker than any other of the family, but I have not ascertained the fact. With 

 the exception of the two species just mentioned, they are all scarce shells, but found occasionally in all parts of 

 the carboniferous series, from the millstone grit to the yellow sandstone, inclusive. 



Brachythyris duplicicosta. Pltil sp. 



Spirifera duplicicosta. Phil. Geol. York. 



Sp. Ch. — Transversely oval, depressed; mesial fold angular; about seven ribs on each side the mesial fold, 

 each of which, at one-third its length from the beak, divides Into three others. 



This species is rendered remarkable by the regularity with which the ribs branch as they approach 

 the margin ; the mesial fold is angular, very prominent at the margin, usually bearing three ribs near the beak, 

 which, by their subdivision, form nine as they reach the front. The width exceeds the length by about one- 

 third, the depth is only equal to half the length of the small valve. Length one incli. 



Brachythyris exarata. Flem. sp. 



Spirifer exaratus. Flcm. Brit. Anim. 



Sp. Ch. — Transversely obovate, cardinal area exceedingly sliort ; about six very broad, flat ribs on each 

 side the mesial fold, which is flattened, but distinctly defined, and has usually three obscure ribs ; length three- 

 fourths the width. 



I believe Dr. Fleming, the author of the highly useful History of British Animals, is the only writer who 

 has noticed this remarkable fossil, and I am not aware that there is any figure of it in existence. It resembles 

 Spirifera rotundata in general size and shape, but is distinguished by its very short cardinal area, which is, in 

 fact, almost an equilateral triangle, its length very little exceeding its width. It is curious that this shell, in 

 common with the Brachythyris planicostata, M'Coy, is usually found without its ventral valve. Length one 

 and a half inches, width two inches. 



