22 APPENDIX. 



The surface is divided into three distinct lobes, forming a roof-shaped mesial fold, with wide flattened sides, 

 gradually tapering as they approach the mesial crest, which is occupied by two, three, or four longitudinal 

 ribs placed close to each other ; the lateral lobes, declining rapidly to the margin, are ornamented by from 

 five to six plaits. The ventral or dental valve presents a deep and wide longitudinal sinus, along the 

 centre of which are situated one, two, or three small ribs, which correspond with those on the crest of 

 the opposite valve. Beak short, greatly incurved, and hardly produced beyond the level of the umbone ; 

 foramen in general inconspicuous ; beak-ridges well defined, leaving a wide flattened space between them 

 and the hinge-line, which last indents to a considerable extent the corresponding margin of the dorsal 

 valve. Dimensions variable : length, 11 lines; width, 10 lines; depth, 12 lines. 



Obs. In 1850, Mr. Walton placed in my hands two examples of a Rhynchonella (PL XVIII, fig. 10), 

 from the Inferior Oolite, near Cheltenham, and which seemed to possess some resemblance both to Rh. 

 decorata (Schlotheim) and to Rh. tetrahedra (Sow.), .nd it was after much hesitation, that I then deter- 

 mined to consider the shells in question as a form or variety of Rh. tetrahedra (Sow.) 



Early in 1854, I was informed by Mr. Lycett, that Mr. Jon<-s, of Gloucester, had collected a number ot 

 similar examples from the Ragstones of the Inferior Oolite of Birdlip, Coopers, and Painswick Hills 

 (Gloucestershire) ; that both in Mr. Jones's and his own opinion, the shell in question "was worthy to rank 

 as a separate species," in corroboration of which the last-named gentleman forwarded for my examination 

 a series of specimens of all ages, measuring from three to twelve lines in length, and which exhibited such 

 marked and constant differences from R. tetrahedra proper, as to have urged upon me to propose for it 

 the distinguishing denomination of Rh. sub-decorata. 



When quite young, with an almost equal length and width, the mesial fold is hardly produced above 

 the level of the lateral lobes (PI. A, fig. 26) ; at the dimensions of about 5| lines in length, A\ in width, 

 and 3-i in depth, the shell exhibits a commencement of that mesial fold so largely developed and cha- 

 racteristic of the adult, (PL XVIII, fig. 10, and PL A, figs. 23, 24, 25). When full grown, the mesial 

 crests generally presents three plaits, the central one being the most elevated, except in those cases where 

 there existed two or four, the central ones then presenting an equal height, the lateral two on a lower level. 

 With age the shell acquires additional width. I have proposed to designate this Rhynchonella by the 

 specific denomination of sub-decorata, from its near approach to some varieties oiR. decorata of Schlotheim, 

 and although it appears to constitute a form intermediate in character between the last and R. tetrahedra, 

 it possesses peculiarities which prevent its being united with either the one or the other. 



In stratigraphical position, it is higher than that of any authentic R. tetrahedra hitherto discovered ; 

 and the present conclusion is founded upon the examination of twenty-four examples from the collections 

 of Messrs. J. Jones, Walton, and Dr. Wright. 



Part III. — Plate XVIII, fig. 10. A full grown example, with three plaits on the mesial crest, from 



the Inferior Oolite near Cheltenham, in the collection of Mr. 

 Walton. 



Plate A, fig. 23. A very adult individual, likewise with three plaits on the mesial crest, from the 

 Inferior Oolite of Birdlip Hill. 

 fig. 24 and 26. Younger examples, with only two plaits; same locality, 

 fig. 25. A specimen, with four plaits (the four examples represented in PL A, are from the 

 collection of Mr. J. Jones, of Gloucester). 



