1856.] WRIGHT UPPER LIAS SANDS. 323 



I have collected retain their ribs in mature age, whilst Ammonites 

 Dorsetensis as constantly loses them, the whirls in old specimens 

 of this species being always smooth. The ribs in A. Parkinsoni are 

 always more sharp and prominent than those of A. Dorsetensis^ and 

 the latter wants the small terminal tubercles which adorn the dorsal 

 ribs in A. Parkinsoni. In the mould of A. Parkinsoni the middle 

 of the back is slightly excavated, whereas in A. Dorsetensis it is 

 rounded. 



Locality and stratigraphical position. — This species characterizes 

 the " Cephalopoda- bed " in Somersetshire and Dorsetshire ; it is col- 

 lected in abundance at the Half-way House between Yeovil and 

 Sherborne, where the largest shells are obtained ; I have found it 

 near Bridport, and at Burton-Bradstock. In its stratigraphical po- 

 sition, therefore, it differs from A. Parkinsoni, which is always found 

 in the upper ragstones of the Inferior Oolite associated with Echi- 

 nodermattty Brachiopoda, and other fossils found only in that Oolitic 

 zone. 



Trigonia Ramsayii, Wright, nov. sp. 



Syn. Trigonia signata, Lycett, Annals and Magazine of Nat. Hist. 

 ser. 2. vol. xii. p. 239. 



Shell very inequilateral, flat, and elongated, the height being small 

 in proportion to the length ; umbones near the anterior side, small, 

 nether prominent nor recurved ; area well developed, lengthened, 

 flattened, marked with transverse lines of growth ; carinse nearly 

 obsolete, represented by smooth elevations. 



Surface of the valves ornamented with twenty nearly equal-sized 

 tuberculated costae ; the anterior and posterior series form concen- 

 tric ridges, and the middle series ai'e undulated ; the tubercles of the 

 costse are nearly of a uniform size, and placed close together on 

 thickened ridges of the shell. 



Length 2\^ inches. Breadth l^f inch. Thickness \-^ inch. 



Affinities and differences. — This shell resembles Trigonia angu- 

 lata, Sow., in the flatness of the area, and in the absence of pro- 

 minent carinas in that region; but diff'ers from it in being more 

 straight and elongated, and in its height being less in proportion to 

 its length : the umbones are likewise smaller, less prominent, and 

 nearer the border. 



It diff'ers from Trigonia signata, Agass., with which it was iden- 

 tified in the absence of tuberculated carinae fj"om the area, in being 

 more compressed, more elongated, and not so high ; the tuberculated 

 costae are more numerous, and their arrangement less regularly con- 

 centric. It is sufficiently distinct from all the other clavellated 

 species of Trigonia. 



Locality and stratigraphical position. — Collected hitherto only 

 rarely in the Cephalopoda-bed of Frocester, The species is dedi- 

 cated to my friend Professor Ramsay, Local Director of the Geolo- 

 gical Survey. 



