﻿no BRITISH FOSSIL TRIGONIiE. 



This figure, so worthless in itself, has been the source of confusions and errors of 

 identification in the Cretaceous Tngonia for upwards of sixty years. One of the most 

 singular and notable of these arrangements is seen in the great work of D'Orbigny above 

 referred to. I am only able to correlate with some doubt Parkinson's fragment of T. rudis. 

 The confusedly scattered tubercles upon the area, and the apparent absence of distinct 

 carinal nodes, associate it better with the large variety of T. dcedalca than with any other 

 species. Professor Morris (' Catalogue,' p. 229) placed T. rudis doubtfully with T. speda- 

 bills. A slight error in Sowerby's delineation of T. dmdalea (' Min. Con.,' tab. Ixxxviii) 

 gives the appearance of rounded nodes upon the marginal angle of the valve, larger than 

 those upon the adjacent rows of pallial varices ; this has been a frequent source of error 

 connected with T. nodosa, or rather with its variety Orhignyana; specimens of this 

 variety have been freely dispersed over Europe, and have been regarded as examples 

 of T. dadalea ; it was adopted as such by Ibbetson and Forbes ('Proc. Geol. Soc,' 

 vol. iv, p. 414), by Fitton (' Quart. Journ. Geol. Soc.,' vol. iii, p. 317), and also by the 

 latter author in his elaborate stratigraphical list of Lower Greensand fossils 

 It is also the 1\ dcedalea of Mantell (' Geology of the South-East of England,' 

 p. 179) and of De la Beche (' Geol, Manual,' p. 287). It may afford some 

 explanation of these errors to mention that Sowerby's original figure of 

 T. nodosa does not accord very closely with any actual known specimen ; the effects 

 of vertical pressure will explain the appearance of flattening at the umbo, the 

 partial exposure of the striated hinge processes, the apparent absence of the angulated 

 costae upon that portion of the shell. ]\Ir. Sowerby's collections of fossils, now in the 

 British Museum, does not contain the original specimen of T. nodosa figured in the 

 ' Mineral Conchology,' and no information concerning it can be obtained ; two very 

 indifferently preserved portions of the Trigonia in the collection are probably the " inside 

 casts " mentioned in his description. 



Agassiz, in common with other palaeontologists, appears to have experienced much 

 difficulty in the determination of this species (' Trigonies,' p. 27, tab. vii, figs. 21 — 23 ; 

 table viii, figs. 2 — 4 ;) the figures upon his plates are named T. nodosa ; subsequently he 

 was led to regard the species of Sowerby as distinct, and described the supposed new 

 species under the appellation of T. cincta. Agassiz grounds the distinctness of T. cincta 

 upon its smaller and more regular varices, upon the greater breadth of its area, and upon 

 the ornamentation of the area, which in T. nodosa appears to be nearly smooth, excepting 

 that it possesses carinal nodes. The condition of the Hythe specimens is such that we 

 should not expect to have the ornamentation of the area preserved ; usually the size of 

 the area is nearly equal to a moiety of the entire smface of the valve, and the rows of 

 varices in Sowerby's figure do not agree very strictly with other Neocomian examples 

 of the same species. The only specimen with the test preserved figured by Agassiz is 

 in an indifferent condition of preservation, the general figure resembles the typical form 

 as exemplified by our specimen from the brown pisolite of Tealby, and also the variety 



