248 



E. ETHERIDGE ON A TRIGONIA FROM THE 



mihi, much resembles T. tenuitextd from the Portland Oolite of 

 Portland, Devizes, Crookwood and Tisbury ; but the shell is more 

 depressed and lengthened posteriorly than T. tenuitexta, and does 

 Dot possess the antecarinal sulcus or space of that species, which 

 occurs in all the known Jurassic Glabrae, and which is so essentially 

 characteristic of the Portland group. 



The ornamentation is that of the Upper-Jurassic Glabra? ; but the 

 escutcheon, which is remarkably large and possesses transverse 

 rugae, agrees with that of the ISTeocomian Quadratae, to which it 

 allies the species. 



All the Neocomian Quadratae have the escutcheon ornamented, 

 and therefore distinct from that part of the tuberculated Jurassic 

 species in which it is plain or unornamented. Again, the Cretaceous 

 Scabrae have also ornamented escutcheons ; but the latter are alto- 

 gether different and far removed from the Jurassic Trigonice. 



I therefore regard this new species of Trigonia as a transition form, 

 tending to connect the Portlandian Glabrae, through its ornamenta- 

 tion, with the Neocomian Quadratae, through the ornamented and 

 peculiar or characteristic escutcheon. Its stratigraphical position 

 at the highest portion of the Jurassic system indicates a zoological 

 change in progress as regards the genus Trigonia, constituting an 

 important feature in the history of the genus, no other example of 

 which, so far as I am aware, is known in rocks representing the 

 Purbeck formation, either British or continental. 



This new species falls under the group or section Glabrae, founded 

 by Agassiz upon very insufficient data, and described by him as 

 being without ornamentation, having no tubercles or costae. The 

 shells of this group are usually " inflated-ovate or ovately-oblong ; 

 and the area is only slightly separated" from the rest of the valve. 



Mesially, or anterior to the position of the marginal carina, there is 

 a smooth space which commences at the apex (umbo) or near it, and 

 gradually widens downwards to the posteal border. This smooth 

 space is more depressed than any other portion of the valves, and is 

 distinctly impressed by lines of growth. 



In this group the " anteal " portion of the valves possesses costae 

 more or less prominent, usually small, closely arranged, and either 

 plain or tuberculated. The number of species belonging to this sec- 

 tion is seven, six of which are Jurassic, the remaining form being 

 Cretaceous (T. excentrica, Park.). 



Our species is closely allied to those forms known in the Portland 

 rocks under the name T. gibbosa, Sow., T. Damoniana, de Lor., T. 

 Manselli, Lycett, and T. tenuitexta, Lycett, and especially to the 

 last named, from which, however, it differs in form and characters. 



Note on the Strata. By the Eev. TV. P. Andrews, M.A. 



Hr. Andrews has drawn up the following description of the 

 Purbeck beds of the Yale of W ardour ; and any of the sections would 

 suffice to read aright the succession and position of the fossil under 

 consideration. 



