GLABRA. 91 



Affinities. Compared with the aUied form, T. Damoniana, the partial disappearance of 

 the ante-carinal space, the smaller, much more numerous and crowded, meandering costse, 

 serve sufficiently to distinguish it. These characters also separate it from other Trigoniee 

 of the same section. 



The magnified figure 1 a represents the umbonal costse constituting a very numerous, 

 minute series, directed horizontally, without interruption, across both the ante-carinal 

 space and the area. 



Siratigraphical position. The specimen figured is from the limestone of the Isle of 

 Portland. A series of specimens more or less imperfect, kindly forwarded to me by Mr. 

 Cnnnington, proves that the species also occurs in the Portland Oolite at Devizes, Crook- 

 wood, and Tisbury, 



Trigonia Beesleyana, Lycett, sp. nov. Plate XVII, figs. 2, 3, 4. 



Shell ovately oblong, depressed, transverse, thin ; umbones antero-mesial, small, 

 depressed, only slightly raised above the superior border; anterior side moderately 

 produced ; its border rounded ; lower border lengthened and curved elliptically, its 

 posteal extremity pointed ; superior border moderately lengthened, nearly horizontal, 

 carved downwards posteally. Each valve is divided into two unequal portions by a plain 

 oblique angle. The escutcheon is of moderate breadth, flattened, but somewhat concave, 

 traversed by very numerous, rounded, regular, depressed, delicate costellse, which pass 

 across its surface horizontally, and are slightly indented transversely by the lines of 

 growth ; a few of the costellae are bifurcated near to the superior border. There is no 

 marginal carina, but a distinct oblique divisional angle, such as occurs in a portion of the 

 Scabrce ; to this latter group it is also allied by the entire absence of an area and by the 

 ornamentation of the escutcheon, the costellae upon which are similar in character or 

 slightly scabrous. The middle portion of the valve comprising nearly the half of the 

 whole surface is plain, and its surface is traversed only by the very delicate lines of 

 growth ; it is also without the oblique depression which usually is characteristic of the 

 Glabra. The anteal portion of the valve has numerous delicate, narrow, smooth, small 

 costae, which are conspicuous at the anteal border ; they pass towards the middle of the 

 valve obliquely downwards; they are slightly waved, and become evanescent ere they 

 have traversed little more than one fourth of the length of the valve ; towards the umbo 

 they are scarcely perceptible. The lines of growth are numerous, unequal, and delicate. 



The internal mould is well preserved, it exhibits very wide-spreading, coarsely striated, 

 dental processes, and owing to the general depression of the valves, there is little convexity 

 excepting near to the umbones ; the muscular scars have but little prominence, and there 

 are no traces of the external ornamentation. I have also succeeded in exposing the 



