150 



SYNOPSIS OF THE CHARACTERS OF THE 



ACTINOCONCHUS PARADOXUS. M'Coy. (PL XXI. fig. 6). 

 Terebratula Roissyi. Verneuil. Bui. Soc. Geol. Fran. 

 The depth of the smooth, central portion is equal to two-thirds its length, measured from the beak to the 

 commencement of the striated portion. The disc has, at its origin, a narrow space, quite smooth, and free from 

 stria; the disc itself has numerous fine, radiating sulci, at equal distances, the spaces between them flat. In 

 general, the central portion is dull, while the disc possesses a slightly lustrous, or shining appearance; length of 

 central portion eight lines, breadth seven lines ; disc projects about three-fourths of an inch beyond the shell 

 on every side, excepting a space at the beak. Since I read the paper above referred to, M. de Verneuil pub- 

 lished a description of this fossil in the Bui. Geol. Soc. de France, under the name of Terebratula Roissyi, 

 stating the beak to be perforated, as in the recent Terebratula, and referring it to the Spirifer Roissyi of 

 L'Eveille ; as I have taken different views on both those points, and had described it so long before, I have re- 

 tained my old name. 



Family TEREBEATULID./E. 



The Terehratulida; may be divided into the following genera : 1. Delthyridaa, M'Coy (Fig. 27), or those 

 species which have a long hinge-line, and a distinct, cardinal area (as in the Delthyrida. or Spirifers), the 

 deltidium fissured in the middle ; all the species of this genus are largely plaited ; 2. Terebratula (Fig. 28) 

 (as here restricted), composed of those species which have the beak truncated so as to form a large circular per- 

 foration ; the deltidiimi only reaches to the anterior margin of the foramen, not extending round it ; all the 

 smooth, perforated species belong to this genus ; 3. Cyclothyris, M'Coy (Fig. 29), formed of those curious 



Fig. 27. Fig. 28. Fig. 29. Fig. 30. Fig. 31. 



species in which the deltidium completely encircles the foramen; 4. Afrypa, Dal. (Fig. 30) (as here re- 

 stricted), composed of those species which are destitute of a foramen and deltidium at all periods of their life ; 

 and, 5. Seminula, M'Coy (Fig. 31), a genus formed for the reception of those little species which have a 

 minute perforation, but want the deltidium. The two last genera are the only ones found in the Palasozoic 

 rocks, and include all the so-called TerebratulcE of these formations. 



Atrypa. Dal. (Restricted). 



Gen. Ch. — Equilateral, inequivalve; dorsal valve imperforate, and without deltidium; no hinge-line, 

 brachial supports simple, flat, triangular. 



The term Atrypa has, of late, been used in so wide a sense, that its restriction, as in the present instance, 

 to the imperforate Terebratulse of the older rocks may be convenient^. The species of this genus, as now 



* Mr. Sowerby makes it include some of the present shells, and also the smooth Spirifers, with a short hinge-line and 

 ■wide cardinal area {Martinia, M'Coy) ; the imperforate Spirifers, without hinge-Une, cardinal area or foramen, but pos- 

 sessing the spiral appendages (At/iyris, M'Coy); and some others. 



