110 



BRITISH SILURIAN BRACHIOPODA. 



about the middle of tlie valve, leaving between them a pear-shaped depression filled by the 

 scars of the occlusor and adjuster muscles. The interior of the dorsal valve has, under 

 the incurved beak, a small hinge-plate, divided in the middle ; each half broadly triangular 

 and depressed along the centre ; these hinge-plates are supported below by a median 

 septum, which extends to a little more than one third of the length of the valve. On the 

 outer side of the hinge- pi ate are excavated the dental sockets, and to the hinge-plate is 

 attached a comphcated system of lamellae, which form the spiral coils, with their 

 extremities directed outwards or towards the lateral margins of the shell; each spiral 

 being composed of about fifteen convolutions. The adductor or occlusor muscular 

 impressions are oval scars close to and on either side of the lower portion of the septum. 

 Ovarian pits are also in some specimens observable on the visceral portion of the interior. 

 Two specimens measured — 



Length 23, width 23, depth 17 lines. 

 „ 16, „ 20, „ 12 „ 



Ohs. I have devoted an entire Plate to the variations in shape assumed by this 

 remarkable species. Fig. 9 is viewed from the beaks, in order to show the two diverging 

 lines, which are in some specimens visible through the transparent shell, and which, 

 commencing at the extremity of the beak in the ventral valve, extend to about one third 

 of its length. These, and the single central line in the dorsal valve, indicate the position of 

 the hinge-plates and septum in the interior. The external surface in all the many specimens 

 I have examined was smooth ; but when the specimen is shghtly decorticated, the fine 

 radiating striae described by Mr. J. de C. Sowerby are observable. The species appears 

 to have been described for the first time about 1815, by J. Sowerby, under the designa- 

 tion of Terehratula obtusa ; and he also at the same time gave a figure of the exterior, 

 and another in which portions of the spiral coils were exposed, so that in reality, were the 

 rules of priority to be strictly adhered to, the shell under description ought to retain the 

 designation of Meristella obtusa, Sow,, sp.^ 



In 1827 the same shell received from Dalman the designation of Atrypa tumida ; and, 



' In vol. xii, p. 5 1 5, of the ' Linnean Transactions,' Sowerby describes his shell in the following words : — 

 " Since I presented a sketch of an Anomia or Terehratula with internal cartilage (it was Spirifera striata), 

 I have received a species of Terehratula of a very different construction, with a spiral cartilage ; and I con- 

 ceive that a figure of this species would be a convenient addition to my former communication, as showing 

 that the spiral cartilage is less confined to shells of a certain external form than might have been expected. 

 Such as I had seen before had straight elongated hinges, and the deeper shell had a triangular foramen, 

 or distant curved beaks indicating it ; the present sketch, figs. 3, 4, represents a shell curved laterally from 

 the hinge, which must be very short ; the beaks are very small, and without any space for a triangular 

 foramen externally ; although within there appears a nearly triangular appendage to the cartilage, which, if 

 not possessed of the outer shell, might indicate a triangular foramen. I presume to lay this sketch before the 

 Society, hoping the subject will meet with attention and investigation when opportunity ofi'ers. The 

 construction of the shells in my former communication would, without the present specimen, have given 

 an idea of the straight hinge being peculiar to those Terehratulce with a spiral cartilage, as agreeing with 



