RHYNCHONELLIDtE. 



143 



incurved, fissure triangular, commencing under the extremity of the beak, and extending 

 to the hinge-Hne; no area or deltidium. The lateral portions of the beak merge 

 gradually into the edge of the fissure; beak-ridges slightly defined, leaving between 

 them and the fissure a rather wide, smooth, convex space. Dorsal valve oval, and more 

 or less gibbose, with a slight longitudinal depression along the middle ; marginal line 

 slightly flexuous, almost straight or gently convex in front. Surface of valves ornamented 

 by numerous simple angular ribs, rarely exceeding two lines in width at their greatest 

 breadth; one or two sometimes bifurcating. The dental plates of the ventral valve, 

 converging in a trough-like shape for half the depth of the valve, then coalesce into a 

 strong median plate, which extends two-thirds or more down the valve. In the dorsal 

 valve two much smaller, separate, longitudinal septa, with a narrow space between them, 

 extend to about half the length of the valve, and to which the socket-plates converge 

 and join, as in the corresponding plates of the ventral valve. Three specimens 

 measured — 



Length 4 inches 9 Hues, width 3 inches 2 lines, depth 2 inches 10 lines. 

 2 „ 4 „ „ 2 „ 2 „ » 1 „ 8 „ 

 1 „ 4 „ „ 1 „ 11 „ „ I „ 4 „ 

 Ods. In his charmingly writt3n book, 'A Summer's Ramble amongst the Hebrides,' 

 that distinguished palaeontologist, Hugh Miller, remarks how preferable it is to hazard the 

 risk of being even tediously minute in descriptions, to incurring the danger of being inade- 

 quately brief in them. But, " alas ! for purposes of exact science, rarely are verbal 

 descriptions other than inadequate." The truth of this is by myself constantly felt ; and 

 as it is impossible to draw up a description fully answering to the shape of every speci- 

 men, the general characters must be more particularly aimed at in the written definition ; 

 and, as much as possible to make amends for my own shortcomings, I have endeavoured 

 to speak to the eye by the means of numerous illustrations. While stating how variable 

 are the forms assumed by this species, I may remind the student that in PL XVI and 

 XVII he will find the principal modifications assumed by F. Knij/ktii,' and I leave to 

 his imagination the task of connecting them by every gradation of shape. F. Knic/Uii 

 was weU figm-ed by James Sowerby in 1813; and his description,^ although somewhat 



1 Thus Sowerby states, " The septa in the flat valve extend to its edge " (this is not correct, for they 

 extend only half way) ; " they are near each other and parallel. The septum in the other valve divides 

 the beak ; the divisions extending to the edge of the shell form a kind of double beak, much resembling 

 the horny part of the toes of a pig's foot, and leave an angular hollow within the curve : the stone that 

 fills this hoUow commonly separates easily, and may at first sight be taken for a third valve of a triangular 

 carinated form. The edge of the deep valve appears to extend over that of the flatter one ; the length 

 of the curve, from the apex to the edge, is often six inches. About half a mile up the River Teme, near 

 Downton Castle, the dell is bounded by two steep rocks, approaching to each other ; and the parts of the 

 land above are level and alluvial, as if a lake had been there before the present chasm in the rock. On 

 the southern rock (a dark-grey limestone) these extraordinary split shells are found, and are situate" 

 about twenty feet above the level of the river. I am greatly obliged to T. A. Knight, Esq., of Downton, 



