﻿can ever arise respecting their identity as a species. With the presumed variety the 

 conditions are altogether different; deprived of the test, the most rehable means of 



118 BRITISH FOSSIL TRIGONIyE. 



exemplified by our figures, it is, perhaps, doubtful whether the variety from the Chloritic 

 Marls and Sands should be named aliformis. The question has engaged my attention 

 fully, aided by the comparison of ample materials. The typical form of the Blackdown 

 and Haldon Greensand is remarkably exempt from any considerable amount of variability, 

 so that, whatever may be the number of specimens or their stages of growth, no question 



comparison does not exist. The Warminster specimens are not only ill preserved, but 1 



they are invariably flattened and sometimes distorted by vertical pressure. Such 

 appears to be the case with all the specimens collected by Mr. Cunnington, who kindly 

 forwarded to me an unusually numerous series for comparison. A varied series of 

 specimens from a similar stratigraphical position near Ventnor affords a nearly similar 

 result, excepting that the fossils, having been enveloped in a finer sediment, have their 

 surfaces occasionally better preserved, and have sometimes, but rarely, escaped com- 

 pression. With such selected specimens, however, occur numerous others, flattened, but 

 appearing to possess the usual attributes of the typical form in a degree suflHciently 

 marked to prevent them from being assigned to a distinct variety. Having due regard, 

 therefore, to the much higher position of the presumed variety, I propose to regard it as 

 such, and to distinguish it by the name attenuata. The typical form is somewhat 

 abundant in the lower beds of the Blackdown and Haldon Greensand, associated with a 

 still more common species of the same group, T. scabricola. It has also been 

 tabulated in lists of Lower Greensand fossils by Ibbetson and Forbes, by Fitton, by 

 Mantell, and by Morris. A few badly preserved examples representing fossils of the 

 same group, obtained in the Lower Greensand at the Isle of Wight, in the Kentish beds, 

 and also near Cambridge, have been brought under my notice; for these and for 

 similar examples from France the reader is referred to the next species, T. Vectiana. 



History, and Comjuirisons loith allied forim of the Scahra. — The figures of T. aliformis 

 given by Parkinson (' Org. Rem.,' vol. iii, pi. xii, fig. 9) and by Sowerby (' Min. 

 Conch.,' vol. iii, pi. 21.5) are excellent representations of the typical form from the 

 Greensand (Whetstone pits) of the Blackdown district. During some years subsequently, 

 Continental cultivators of palaeontology, in the absence of the means of comparison which 

 are now possessed, assigned to Parkinson's species several allied forms of T)-igonia 

 from various localities and stratigraphical positions. The general absence of sufficiently 

 definite descriptions of the several features which characterise the Trigonia Scabra, in 

 these and other authors, has induced me to omit notice of such descriptions when they 

 are not accompanied by illustrative figures, unless they are in other respects sufficiently 

 verified. As T. aliformis has been believed to occur at various localities in each of the 

 four continents, the comparisons, in some instances referring to figures of fossils, the originals 

 of which are unknown to me, require much critical care in estimating them. 



The figure and brief description of Trigonia thoracica, Morton (' Synopsis Org. Rem. 

 of the Calcareous Group of Alabama, United States,' p. 65, pi. xv, fig. 13, 1834), 



