CARBONIFEROUS FOSSILS OF IRELAND. 151 



restricted, are very numerous and extremely various in tlicir form, some being transverse, others elongate ; some 

 having a smooth surface, and a still greater number plaited longitudinally. 



The genus Atri/pa, according to our view, includes all those species of Terebratuliform shells which are 

 without a distinct perforation in the dorsal valve ; and of course as wanting the perforation, they also want the 

 doltidiura; they were, therefore, probably, free shells, another very interesting peculiarity, in which they differ 

 from the Terebratula. 



I have been enabled to discover the internal structure in many of the species, and find it to be much more 

 simple than that of the more typical groups ; the supports of the arms assume the form of two broad, fiat, trian- 

 gular processes, narrow at their origin near the beak of the ventral valve, and increasing in breadth towards 

 their free extremities, where they are obliquely truncated ; they are not recurved in any of the species, nor is 

 there any connecting process between them. The arms were probably free, and capable of being protruded for 

 the greater part of their length. This group is generally confounded with the imperforate Spirifers, forming the 

 genus Athyris, M'Coy, but is at once distinguished by the simplicity of its internal supports, these parts being 

 very long, and spirally coiled in the latter genus. Professor Phillips' genus Cleiothjris appears to include 

 them both. 



The genus Atrypa is exclusively confined to the Paleozoic rocks. 



Atrypa acuminata. Mart. sp. 



Anomites acuminatus. Martin, Pet. Derb. — Terebratula acuminata. Soie. Min. Con Terebratula acuminata. 



Phil. Geol. York. 



Sp. Ch. — Tetrahedral, ventral valve very much elevated, obtusely keeled in the middle ; dorsal valve 

 small, concave ; margin sharp, forming a very acute sinus in the front. 



This species is subject to an extraordinary degree of variation in its form. The most usual and typical 

 Fig 32. form is that described in the above specific character, but others occur in which the sinus in 



the front is obtuse and plaited, and others with lateral plaits are not uncommon. Some spe- 

 cimens are depressed, as the T. platyloba, Sow. ; all the varieties have the surface smooth to 

 the naked eye, but minutely striated under the lens. The internal structure is similar to 

 that of the A. hastata, but the supports are smaller in proportion to the size of the shell (see 

 fig. 32). This is a local species in Ireland, but where it does occur it is usually very abundant. 

 Length one inch three lines, width one inch five lines. 



Atrypa angularis. Phil. sp. 



Atrypa primipilaris. Sow. Geol. Trans, (not of Von Buck). — Terebratula angularis. Phil. Pal. Fos. 

 Sp. Ch. — Pentagonal, wider than long ; mesial fold large, prominent, sharply defined, flattened; surface 

 radiated with numerous, irregular, branching stria. Length three and a half lines, width four lines. 



Atrypa anisodonta. Phil. sp. 



Terebratula anisodonta. Phil. Pal. Fos. 

 I have considered as this species, those specimens agreeing with T. pleurodon in general form, but having 

 the plaits rounded, and becoming obsolete towards the beak. 



Atrypa aspera. Dal. 



Terebratula aspera. Schlot. — Atrypa squamosa. Sow. Geol. Trans. 



Sp. Ch. — Outline from transversely oval to longitudinally obovate, dorsal valve very convex; ventral 

 valve less so, beaks prominent; surface radiated with from fourteen to eighteen strong, rounded ribs, with rather 

 distant, concentric rows of large, scale-like laminse of growth. 



