20 BRITISH FOSSIL TRIGONLE. 



" Trigoniae of the Coral Rag ;" he also states that he has seen specimens from the 

 Calcareous Grit of Weymouth, and even expresses doubts as to the real type of T. 

 clavellata. The specimens of T. Bronnii from Glos (Calvados) are much smaller than the 

 T. clavellata of Weymouth ; in all the specimens which I have examined the escutcheon 

 has greater breadth and is shorter ; the posterior extremity of the valve is broader and 

 less pointed ; adult specimens have the rows of tubercles less numerous, and their general 

 direction is more horizontal ; their irregularity in size and arrangement is also very 

 conspicuous when compared with T. clavellata. 



Another very large clavellated species from the Lower Calcareous Grit of Yorkshire 

 and Oxfordshire has sometimes been mistaken for it ; this is the T. triquetra, Seebach, 

 for which the reader is referred to the description. 



A near ally to T. clavellata is T. perlata, Ag., of which a great profusion of specimens 

 have been obtained in the Coralline Oolite of Pickering, for which also see the descrip- 

 tion. 



In Britain T. clavellata has occurred very abundantly in layers of the Lower 

 Calcareous Grit formation in scars, and in the cliff at Sandyfoot Castle, near Weymouth ; 

 also in the same formation in Wiltshire and near to Filey Point, Yorkshire. Our figures 

 represent its usual dimensions. 



Trigonia Voltzii, Ag. Plate X, figs. 1, 2. 



Trigonia Voltzii, Agassis. Trigone'es, 1840, p. 23, pi. ix, figs. 10 — 12. 



— — Oppel. Juraformation, 1856— 1858, p. 719, No. 88. 



— clavellata, Morris. Catal., 1854, p. 228 (pro parte). 



Few Trigonige have been the cause of so much perplexity and doubt as the present 

 form, the result of the very unsatisfactory figure given by Agassiz, and also of the 

 insufficiency of his description ; so obscure, in fact, has this species appeared to be, that 

 the greater number of authors have been contented to ignore it altogether. The figure 

 in the memoir of Agassiz represents the internal mould of a clavellated Trigonia, some of 

 the tubercles of which are impressed upon its surface, and the only distinctive character 

 that can be ascertained from it is that the form is more elongated than T. clavellata. 



D'Orbigny (' Prodrome,' 2, p. 51, makes T. Voltzii a synonym of T. muricata, 

 Goldfuss. 



Oppel (' Juraformation', p. 719) has a lengthened note upon T. Voltzii, which proves 

 that he was well acquainted with our species, of which he had himself collected several 

 specimens from the Kimmeridge Clay of Boulogne ; but he does not appear to have been 

 so certain with regard to the true type of T. clavellata, and he therefore gave no further 



