RHYNCHONELLA. 65 



Genus — Rhynchonella, 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, rarely spiny ; valves 

 articulating by means of two teeth in the larger and corresponding sockets in the 

 imperforated valve ; apophysary system in smaller valves composed of two short, flattened, 

 and grooved lamellae, separate, and moderately curved upwards, attached to the inner 

 side of the beak of smaller valve, and to which were affixed the free spiral fleshy arms ; x 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. In Part I, I have mentioned the reasons for not admitting M. D'Orbigny's 

 genus Hemithiris, established on Rynchonella psittacea, Wilsoni, &c, from a conviction, 

 that these shells correspond in all essential characters to the genus Rhynchonella ; internally 

 their organisation is similar, the muscular impression and short calcareous process for the 

 support of the free fleshy arms being the same in these as well as in all the species of 

 this extensive genus, whether Recent, Cretaceous, Oolitic, or Palaeozoic. 2 The external 

 shape and character of the different species is, on the contrary, so variable and perplexing, 

 that in many cases it seems almost impossible to trace a line of demarcation between 



1 See Professor Owen's 'Anatomy of T. psittacea,' Trans, of the Zool. Soc, vol. i, 2d part. 



2 Within the last few years, authors seem to agree, as to the propriety of adopting a separate genus for 

 those plaited TerebratulBe, the calcareous appendages of which are formed of only two small, short, curved 

 lamella, to which are attached the free, fleshy arms, as in T. psittacea, loxia, vespertilis, octoplicata, fyc. 



M. D'Orbigny, in 1847, admitted Fischer deWaldheim's genus Rhynchonella, giving the date 1825, and 

 therefore unacquainted with a prior paper by that author, entitled 'Notice sur les Fossiles du Gouvernement 

 de Moscou servant de Programme pour inviter les Members de la Soc. Imperiale a la Seance publique du 

 26 Oct., Moscou, 1809,' wherein will be found the first descriptions and figures of the genus Rhynchonella. 



Professor King, in Part III, of his valuable and interesting 'Monograph of the Permian Fossils,' 

 states, " Reverting for a moment to the types named by the celebrated oryoctographer of Moscow, I would 

 ask, is anything satisfactorily known respecting Trigonella atoma ? and Rhynchonella loxia ? Has any one 

 been able to identify these shells? "What formation do they belong to? And what are their localities, 

 &c. ?" Most of those questions may be answered; but as the paper above alluded to (1809) is little known 



K 



