CLASS II. BRACHIOPODA. FAMILY 2. ADHyERENTIA. 183 



THECIDIUM, Defrance. 



Testa ovata, parva, inaequivalvis, subaequilateralis, terebratulifornns, im- 

 perforata, valva concava affixa. Valva inferior processibus duobus 

 cardinalibus, internis brevibus ; area irregulariter subtrigona ad 

 umbonem acutiusculum extensa Valva superior planiuscula, basi 

 externe appendicuKi brevi obtusa ; interne processibus duobus late- 

 ralibus cardinalibus, ligamento nullo. 



The Thecidia were separated from the Terebratuke by Defrance, and 

 appear to have escaped the particular notice of Lamarck ; they may, 

 however, be reckoned of much importance in the classification on ac- 

 count of their shells being exactly intermediate between those of the 

 two families of the Brachiopoda. They resemble the Terebratulce not 

 only in shape, but in having the cardinal processes, whilst they differ in 

 being imperforate, and therefore destitute of any pedicle or tendon. 

 This genus is also one of considerable interest to the geologist ; in fact, 

 at the time of its first introduction by Defrance it was known only in the 

 fossil state. 



The shell of Thecidium may be described as being ovate, small, in- 

 equivalve, nearly equilateral, imperforate, and attached by the concave 

 valve, which is furnished with two short cardinal processes, and a rather 

 irregularly-triangular area extending to the point of the umbo. The 

 upper valve is flattish, and has a small, short, obtuse appendage at the 

 base ; it is also provided with two lateral cardinal processes, closely 

 interlocking with those of the lower valve ; the hinge, however, as in all 

 the Brachiopoda, is entirely destitute of any ligament. 



The interior of the shell of Thecidium presents a new and very singular 

 appearance, which we have not noted in our generic description ; the in- 

 ternal configuration of the upper valve is peculiar to this genus of Bra- 

 chiopoda, and appears to depend upon some peculiarity unknown to us 



