CLASS II. BRACHIOPODA. FAMILY 1. TENDINOSA. 181 



This genus may also be said to have been introduced by Bruguiere in 

 the plates of the ' Encyclopedie Methodique,' and from the extent of its 

 geological relations it has always been one of considerable interest ; it is, 

 in fact, richer in fossil species than any other. The Terebratula; were as- 

 sociated by Linnseus and his followers with the Anomice on account of the 

 resemblance of their mode of attachment, namely, by means of a muscular 

 tendon passing through an orifice in the shell ; in this genus, however, it 

 is the upper valve that is perforated, and there is no calcareous deposit 

 at the extremity of the tendon as in Anomia ; the fibrous composition of 

 it, indeed, rather suggests a modification of the byssus of the Aviculacea. 

 The Terebratula? have a shell of very different construction from that of 

 the Lingula ; instead of the valves being equal and held together by mus- 

 cular attachment only, there is a considerable inequality in them ; each 

 valve is provided with one or more dentiform processes, by means of 

 which they interlock so closely with each other as seldom to be separated 

 without breaking. In some species these processes are much less fully 

 developed, and the lower or imperforate valve is furnished with a curious 

 testaceous apparatus in the shape of a flattened hoop ; this internal ske- 

 leton, if it may be so called, is somewhat elastic, and is destined to give 

 attachment to the muscular stem of the brachia or arms. 



The shell of Terebratula may be described as being inequivalve, inequi- 

 lateral, oval or orbicular, adhering by a rather short fibro-gelatinous ten- 

 don. The upper valve is always produced at the apex, often curved 

 inwards, and perforated for the passage of the tendon ; it has also a 

 single dentiform hinge process on each side. The lower valve is fur- 

 nished with two slender shelly processes, generally recurved and vari- 

 ously bent, anastomosing within the body of the animal. There are 

 several muscular impressions, but they are very indistinctly marked. 



Examples. 



PL CXXVI. Fig. 1 . 



Terebratula sanguinea, Sowerby, Genera of Shells, No. 15. 

 Anornia sanguinea, Dillwyn. 



