of a few Brachiopoda. 251 



Genus Rhyxchonella, Fischer, 1809. 



Animal small, generally attached to submarine objects by- 

 means of a pedicle issuing from the foramen placed under the 

 beak of the larger valve. Shell inequivalve, variable in shape, 

 wider than long, or longer than wide, circular or elongated ; 

 valves more or less convex, with or without a longitudinal mesial 

 fold and sinus ; beak acute, slightly or greatly recurved ; no true 

 area ; foramen variable in its dimensions and form, placed under 

 the beak, exposed or concealed, entirely or partially surrounded 

 by a deltidium in two pieces, at times extending in the shape of 

 a tubular expansion, at other times rudimentary, the foramen 

 being completed by a small portion of the umbo. Surface 

 striated, plaited, or costellated, rarely smooth ; structure fibrous, 

 unpunctuated, sometimes spiny ; valves articulating by means of 

 two teeth in the larger and corresponding sockets in the im- 

 perforated valve ; apophysary system in smaller valve composed 

 of two short, flattened and grooved lamellse, separate and mode- 

 rately curved upwards, attached to the inner side of the beak of 

 smaller valve, and to which were affixed the free spiral fleshy 

 arms* ; a small central longitudinal septum more or less elevated 

 is seen to extend along the bottom of the smaller valve from 

 under the beak to about half or two-thirds the length of the shell, 

 and separating the muscular impressions visible on either side. 



Obs. We shall first inquire if these lamella supporting the free 

 fleshy arms differ in the proposed genera above enumerated, taking, 

 to begin with, the genus Hemithiris, of which M. D'Orbigny 

 considers the recent T. psittacea to be the type, and in which he 

 includes a great number of Palaeozoic species, of which T. Wilsoni 

 and sub-Wihoni, D'Orb., may be mentioned as examples; but, 

 strange to say, no representatives are stated to have lived during 

 the Tertiary, Cretaceous and Jurassic periods, the genus having 

 abounded, according to M. D'Orbigny, in the Palaeozoic epoch, and 

 after having become extinct for countless ages, reappeared iu the 

 shape of a solitary form in the recent state (R. psittacea). R, 

 spinosa and senticosa, at one time placed in Hemithiris, were 

 soon removed to form a distinct genus under the name of 

 Acanthothiris ; the genus Rhynchonplla being stated not to occur 

 in the recent period, but to have abounded in the Cretaceous, 

 Jurassic, and in the Palaeozoic period. From a minute examination 

 of a number of the species distributed into these three genera by 

 M. D'Orbigny, I cannot discover any important variation in the 

 position or shape of the curved lamellse above described, which 

 are similar, and in all more or less developed; the mesial 



* See Prof. Owen's nnatociy of T. psittacea. Trans, of the Zool. Soe, 

 -vol. i. 2nd part. 



17* 



