﻿4 BRITISH FOSSIL TRIGONIJL. 



their surfaces upon different planes, so that at their junction there is a divisional angle 

 which passes obliquely from the apex of the valve or umbo to the lower and posterior 

 extremity ; the divisional angle commonly takes the form of a ridge ; this is the marginal 

 carina, and the portion of surface posteal to it is the area. At the superior border of the 

 area is another ridge or carina, which, like the other, originates at the apex, and forms a 

 curvature posteally to meet the corresponding carina of the other valve. This is the 

 iimer carina, and the space enclosed between the inner carinas of the opposing valves is 

 usually heart-shaped, or more lengthened and lanceolate. This is the escutcheon, which in 

 its turn embraces at the upper and anteal portion of its border the nymphal or ligamental 

 plates and the ligament itself, which is also not unfrequently preserved. The area is 

 usually divided into two portions longitudinally by a slight furrow, and the superior 

 portion is frequently more depressed than the other ; a slight ridge or row of tubei'cles 

 may border this median fitrrow, and, when present, it constitutes the median carina. The 

 anteal portion of the valve, or that adjacent to the pallial border, is invariably more or 

 less ornamented with ribs, which are either plain or tuberculated ; the area also usually 

 has some kind of ridges ; but, in lieu of designating the former as pallial costae and those 

 upon the area as cardinal costae, I prefer, for brevity, to speak of the former as codce and 

 the latter, which are usually smaller, as costella. One other feature common to the 

 surface of all, and therefore constituting a portion of the attributes of the genus, may also 

 be adverted to, more especially as it has usually remained altogether unnoticed. I 

 allude to the epidermal graiuilated tegument ; in common with the whole group of the 

 Anatinidae, and with certain other genera of Conchifera, including the allied genus Opis, 

 the granules are arranged in lines perpendicular to the pallial border. They are more 

 minute and closely arranged than is usual in other genera ; in the Trigonice clavellata 

 more especially the lines of granules can only be discovered by the aid of considerable 

 magnifying power; in this particular the distinction between the CJavellafce and Costata is 

 strongly marked, the latter having the lines of granules so much larger and more widely 

 separated that they may frequently be distinguished by the unaided vision. 



The seven sections into which Agassiz divided the fossil Trigoniae are all exemplified 

 by British species, and to those I propose to add an additional section, the Byssiferce. The 

 sectional distinctions are founded upon the figure of the shell and of its proportions, 

 of the ornamentation upon the pallial or costated portion of the surface, of that upon the 

 area, and of that upon the escutcheon ; lastly, upon the consideration conjunctively of all 

 these parts in any particular group in comparison with the corresponding parts pertaining 

 to other groups of the same genus. An attentive examination will, it is believed, 

 exhibit fully the natural affinity of the various species comprising these separate groups 

 or sections, and also the changes that the genus underwent in geological time during the 

 great epoch of the Secondary Formations. 



