UNDULATiE. 75 



Stratigrapldcal position and Localities. Our specimen was obtained by Mr. Walton, 

 of Bath, in the Cornbrash of Chippenham ; the type specimen was fomid by the late 

 Professor Edmund Forbes in a stratum of soft crumbling yellowish limestone and shale 

 beneath the Oxford Clay at Loch Staffin, Isle of Skye, associated with Testacea, which 

 are for the most part estuarine or fluviatile forms ; the geological position is probably not 

 very different from that of the Chippenham Cornbrash, but modified by peculiar local 

 conditions.^ 



Trigonia detrita, Terci- and Jdur. PI. X, figs. 3, 3 a, 4. 



Trigoia deettrita, Terquem et Jourdy. Monog. de 1' Etage Bathoniea dans le 



Departement de la Moselle ; Mem. Soc. 

 Geol. de France, deux, se'r., torn, neuv,, 

 1869, pi. xii, figs. 1 and 2. 



Shell ovately trigonal, moderately convex ; umbones placed within the anterior third 

 of the valves, moderately elevated, only slightly recurved ; anterior side very short, its 

 border curved elliptically with the lower border; superior border straight, sloping 

 obliquely downwards and forming an obtuse angle with the posteal border of the area. 

 Escutcheon lengthened, concave, its superior border somewhat raised. Area of moderate 

 breadth, flattened, having a well-defined mesial furrow ; it is transversely rugosely 

 plicated and bounded by two small elevated, tuberculated, or rather closely arranged, 

 plicated carinse, which are formed by enlarged continuations of the transverse rugae upon 

 the area. The other portion of the shell has numerous, very closely arranged, plain, 

 rounded costse ; the few first-formed are regular and almost horizontal, succeeded by rows 

 more oblique or directed obliquely downwards from the anterior border ; posteally they 

 form a sudden flexure somewhat downwards and then turn upwards perpendicularly to 

 the carina ; at first the posteal angle is but small, but it regularly increases with the 

 succeeding costae, so that the few last formed have their posteal portions somewhat dis- 

 united ; they become slightly nodulous and, forming each a right angle, pass per- 



' The association of the generic forms of Testacea in the Hebridean deposit described in the 

 Memoir quoted and depicted upon the plate which accompanies it, is curious and instructive, especiallj' 

 ■when compared with the habitats of two of the living Australian Trigonia. T. tripartita was found 

 in the Loch StafBn shale with a Perna and an Ostrea, and with ten other species, belonging to the 

 fluviatile genera, Cyrena, Potamomya, Unio, Neritina, and Hydrobia. That this association was not 

 accidental may be inferred from the following analogous facts. The recent Trigonia Lamarckii occurs in 

 Sydney Harbour, partially buried in black mud, and also in the Paramatta River, within reach of the tide ; 

 in like manner the larger T. pectinata is found in Launceston River, Tasmania, in similar black mud, 

 exposed to the alternating influences of fresh and of tidal brackish water. Any section of these Australian 

 deposits may, therefore, be expected to expose the Trigonice associated, as at Loch Stafiin, with the fluviatile 

 or estuarine testacea with which they lived and were buried. 



