66 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



them, or to draw up a description answering to all the variations, which are, doubtless, 

 due to local and other accidental circumstances : we often find specimens of the same species 

 very convex and gibbous, while others are comparatively depressed ; the number of the 

 plaits is also most variable, some shells being ornamented by a greater or less number ; 

 1, 2, 3, 4, 5, 6, &c, forming in different examples of the same species, a more or less 

 defined or elevated mesial fold. The beak is likewise in some nearly straight, exhibiting 

 under it the foramen entirely or otherwise surrounded by the deltidium, while in other 

 specimens no trace of the foramen and deltidium is visible, from the beak becoming so 

 incurved, as almost to touch and overlie the umbo. The dimensions and form of the false 

 area situated between the beak ridges and the hinge margin is also variable, and indents 

 to a less or greater degree the hinge of the smaller valve. It is, therefore, only by 

 combining certain general characters that we can separate certain forms of Rhynchonella, 

 and it is very unsafe to establish new species on the simple inspection of one or two 

 specimens, unless these present characters so marked and peculiar as to make confusion 

 impossible. Many hundreds, I may say thousands, of British and Foreign Rhynchonella? 

 have passed under my examination, and although I have spared no trouble in comparison 

 and research, some of these have not been settled in a manner quite satisfactory to myself, 

 such as B. subtetraedra, Lycettii, lacunosa, Morierei, and, perhaps, one or two others. 



from its great rarity, I think it desirable to insert a short account of its contents, especially as it will 

 explain the origin of a most important genus in the class of Brachiopoda. 

 Fischer divides his family of Terebratulae into four Divisions : — 



I. The Terebratula, the edges of which are smooth and not plaited, ex. Ter. ovata (Fischer), 

 Ency., pi. 239, fig. 2, T. scabra, &c. 



II. The Terebratulae, the edges of which are plaited, ex. Ter. novem-plicata (Fischer), and 

 Ter. octoplicata; this last the author figures, and it is a specimen of Spirifer Walcotti, and is stated to be so 

 in his other Work, ' Ortyctographie du Gouv. de Moscou,' 1830 — 37, p. 41. 



III. The Trilobated Terebratulae, Genus Trigonella. 



" The margin of the trilobated Terebratulae presents a considerable displacement in the middle, the 

 result being a division into three lobes ; the impression of the middle is either smooth or striated. I dis- 

 tinguish these striatums by parts (ro/ujj), so that one recognises at once, by the name of the species, that they 

 belong to the trilobated Terebratulae, where the contour of the edges is not on the same level." 



Of this proposed genus, Fischer names five types ; the first is Ter. atoma, (Fischer, referring to ' Knorr. 

 Petref.' vol. ii, p. 1, B. iii, fig. 6,) but on examining this reference, I am at a loss to know what the type 

 Ter. atoma is, as B. iii, fig. G, does not look like any form of Brachiopod, and can be of no use. The next 

 is T. bitoma, stating that Lister's fig. 7, tab. 450 a, bears much resemblance to his type ; this also I cannot 

 distinguish, that author's figure being so imperfect. Fischer's next two types, T. pentatoma and tritona, 

 are figured by himself, but are specifically undeterminable from the vagueness of the illustrations ; they be- 

 long, however, to that section, with the two small calcareous bent processes, as in T. psittacea; the last- 

 named, or the fifth type, T. polytoma, is a Spiri/er. ' Ency. Meth.,' tab. 244, fig. 4 " b . 



IV. Rhynchonella, p 35. 



" The Terebratulae, the medial edges of which are so elongated as to assume the form of a beak. The 

 extremity of the beaked edge being on the same level with the foramen. These shells doubtless form a distinct 

 genus, characterised as follows : Shell bivalve, regular, with unequal valves, fixing itself bij means of a 



