128 



SYNOPSIS OF THE CHARACTERS OF THE 



true slielly support in the middle as the hollow ; I am led to this conclusion from having myself succeeded in 

 removing this crystalline coating in many species, among others those figured by Mr. Sowerby, and in every in- 

 stance I have found the true spiral process to be perfectly flat, and not as thick as the thinnest writing paper, 

 perfectly resembling, in fact, the (in my mind) analogous processes in the living Terehratulce. The negative 

 argument against their being the arms, properly so-called, is, that in no single instance have these organs been 

 found in any of the Brachiopoda of the newer rocks; even where the impressions of the fleshy arms of Cepha- 

 lopoda have been preserved, none of the TerebratulidcB (which we know had those arms) shew them in the 

 fossil state, but they all exhibit the variously modified, shelly support, preserved in exactly the same manner as 

 tlie external shell. It is only in this group that they assume a spiral form. 



The family DelthyridcB appear to be divided into the five following natural genera: 1. Spirifera, Sow., 

 (fig. 17), composed of those longitudinally-ribbed species, in which the hinge-line is equal to, or exceeds the 

 width of the shell, the cardinal area with parallel sides, the cardinal teeth of the ventral valve large, spirally 

 rolled, and having a triangular foramen beneath the beak of the dorsal valve. 2. Martinia, M'Coy (fig. 18), 



Fig. 17. 



^.ai^^T^Ja.,.^. 



Fig. 18. 



Fig. 19. 



Fig. 20. 



Fig. 21. 



or the smooth Spirlfers in which the hinge-line is less than the width of the shell, and the cardinal area 

 triangular. 3. Athyris, M'Coy (fig. 19), in which there is no vestige of either foramen, cardinal area, or 

 hinge-line. This remarkable genus is frequently confounded with those shells usually named Terehratula, 

 in the older rocks, but is distinguished by the large, spiral appendages, which are wanting in the other group. 

 4. Brachythyris, M'Coy (fig. 20), in which we find the longitudinally-ribbed surface of Spirifera, united with 

 the short hinge-line oi Martinia. 5. Orthis, Dal. (fig. 21), in which there are no spiral appendages, the hinge- 

 line and striffi frequently spinose (as in Leptana), and the cardinal area common to both valves, and its 

 sides inclined towards each other at its angles ; dorsal valve smallest. 



Spirifera. Sow. 



Gen. Ch. — Shell transversely elongate; hinge equalling or exceeding the width of the shell; surface ribbed 

 longitudinally ; cardinal area with the sides parallel, or nearly so ; a triangular foramen beneath the beak of 

 tlie dorsal valve ; dental lamellae short, diverging ; cardinal teeth of the ventral valve very long, spirally rolled. 



This well known genus appears to have three minor or subgeneric types of form ; one of them is Spirifera 

 proper, as above defined; 2. Cyrtia, ofDalman, in which the cardinal area is exceedingly wide, triangular, and 

 the beak of the dorsal valve not incurved ; 3. Fusella, M'Coy, including those little shells allied to the Spi- 

 rifera f us if ormis and rhomhoidea of Phillips, Geol. Yorkshire, having a regular fusiform outline, with a very 

 wide and hollow cardinal area, the sides of which are parallel. 



Spirifera aperturata. ScJilot. sp. 



Terebratula aperturata. Schlot. 7 — Spirifer aperturata. Phil. Pal. Fos. 



Sp.Ch. — Semicircular, gibbous; mesial fold large, rounded, sides rounded, cardinal angles mucronate; 

 sides with coarse, rounded, equal ridges, mesial ridge, and hollow, with fine, sharp, longitudinal ridges, half as 

 thick as those on the sides. 



The Eifel specimens of this shell have more numerous radiating ridges than the Devonian or Irish exam- 

 ples (wliich are identical), and have a more marked difference in thickness between the mesial and lateral ribs. 



