CLASS II. BRACHIOPODA. FAMILY 2. ADILERENTIA. 187 



pro transitu tendinis adhaesionis, lamina longitudinali obtusiuscula 

 intiis terminate. Impressiones musculares in utraque valva quatuor, 

 quarum duae majusculse, subcentrales, approximatse, dua? minores, 

 posticse, submarginales, distantes. Cardo nullus. 



The present genus was instituted by Lamarck for the purpose of distin- 

 guishing a true Brachiopodous mollusk unknown to early writers ; the 

 upper valve of its shell had been described by Midler and Gmelin as a 

 Patella, an error, however, which cannot be wondered at, when it is 

 known that in this class of Mollusca the valves are not united by any 

 hinge ligament. The Orbiculse are peculiar in the composition of their 

 shells, which are much more horny than calcareous ; and they differ also 

 in their mode of attachment. Although fixed immediately to marine 

 substances one upon the other, they do not adhere by any part of the 

 shell, but by means of a cartilaginous tendon passing through a longitu- 

 dinal fissure in the lower valve. This fissure is situated in the centre of 

 a small oval depression, and the organ of adhesion, after passing through 

 it, immediately expands, so that the edges of the slit become concealed. 



Another genus, Discina, was at one time proposed by Lamarck ; but 

 upon the publication of Sowerby's Memoir on the Brachiopoda in the 

 ' Transactions of the Linnean Society,' it was abandoned. In this paper 

 Mr. Sowerby successfully shows that the DiscineB are nothing more than 

 young Orbiculee. 



The shell of Orbicula may be described as being horny, orbicular, 

 somewhat irregular, inrequivalve, and rather compressed ; the upper 

 valve is patelliform, the lower flat. In the centre of the latter is a 

 small oval depression, with an oblique fissure running through it for the 

 passage of the tendon of adhesion, and anterior to this is a flat, rather 

 obtuse longitudinal plate projecting into the interior of the shell. There 

 are four muscular impressions in each valve ; two of them are rather 

 large, approximating near the centre, and the smaller pair are posterior, 

 submarginal, and rather distant. There is no hinge. 



2 b2 



