85 



they amount in number of species, in the opinion of our talented 

 colleague, Mr. S. P. Woodward, to three only, and it is first in 

 Jurassic strata, that they make their appearance in any remarkable 

 number or variety. 



Taking into consideration this fact, with that of the universally 

 admitted variety of forms attributable to one species : to those who 

 have interested themselves in the theory of transmutation, as originally 

 propounded by Lamarck, subsequently by the Author of the Vestiges, 

 and since, more practically by Darwin, in his Treatise on the 

 Origin of Species, the elaboration of figures, and the minute details 

 here presented, although apparently uselessly repeated, may yet 

 assume an aspect of interest which they could not otherwise possess. 

 Ostrea Interstriata (Plicatula, of Emmerich,) of the White Lias, and 

 the small oyster, which covers the slabs of Lower Lias at Wainload, 

 Westbury, Penarth, &c. associated with Modiola minima, recognized 

 by BucKMAN and other local writers, as Ostrea Liassica, are the earliest 

 known to us in this district. Distinctly Gryphoid forms occur con- 

 siderably higher in the series, and are most abundant from the zone 

 of Ammonites Bucklandi, to that in which it is supposed to be replaced 

 by Gryphaea obliqua ; but any one who has carefully examined these 

 in considerable numbers, and can therefore fully appreciate the infinite 

 diversity of form which they assume, rendering the determination of 

 the differences between Oysters and Gryphites exceedingly perplexing, 

 may possibly, in the sequel, feel disposed to adopt the suggestion of 

 QuENSTEDT, that Ostrea Liassica may really be the ancestral precursor 

 of the species under consideration. As the true relations of these 

 forms can only be properly illustrated by examples, we must now 

 refer to figures of the specimens selected for that purpose. 



Figures 1, 1% 1^, Plate 1, represent specimens of ordinary type 

 from Purton, near Sharpness Point. 



Figures 2, 2\ 2'', Plate 1, are half grown, and 3, 4, 4», more 

 advanced forms, in neither of which is the beak obliquely incurved, 

 but is very nearly central, — the lateral lobe is scarcely traceable in 

 either of these specimens, while it will be observed that the smaller 

 valves differ much in character. On comparing figures 2 and 4 with 

 fig. 3, that of 3 is seen to be exceedingly massive, composed also 

 of very thick plates, and rising prominently above the margin of the 

 lower shell, whilst those of figures 2^ and 4*, being formed of 

 thin laminae, are concave, and, as seen in profile, with scarcely 

 any portion of them visible above the margin of the lower valves. 

 All these are from the Ammonites Bucklandi bed at Purton, where 

 they occur in immense numbers, and in the best possible state of 



