﻿COSTATA. 



161 



Wide concave area and escutcheon. 



Delicately sculptured small carinae and intercarpal costellse. 

 Small horizontal costse with their anteal undulation. 



During many years only two examples of this form, from the Cotteswold Hills, have 

 come under my notice; and, in the absence of all other information, frequent comparisons 

 with Inferior Oolite examples of the Costatce were made in the expectation that con- 

 necting forms might be found tending to unite it with them, but without result. At 

 length five examples of T. temicosta were placed in the British Museum from the 

 Inferior Oolite of Bradford Abbas ; subsequently various specimens in differing con- 

 ditions of preservation were kindly forwarded to me by Professor Buckman, from the 

 same locality ; Colonel Mansel Pleydell has also contributed a small specimen obtained 

 by him in the Inferior Oolite at Walditch near Bridport. Comparisons of these 

 materials have removed all doubts of their distinctness from others of the same section, and 

 justified the separation which had been claimed by me for it in the year 1853. 



Position and Localities. At Walditch, two miles from Bridport, the Inferior 

 Oolite is seen to rest upon the Midford or Supra-liassic Sands. At Bradford 

 Abbas, T. temicosta occurs in a single bed from three to five feet thick, termed by 

 Professor Buckman the Cephalopod-bed, from the very numerous and finely preserved 

 species of Inferior Oolite Ammonites which it has produced ; it has also yielded a pro- 

 fusion of other Molluscan forms ; the associated Trigonia consist of two varieties of 

 T. costata, two varieties of T. striata, a variety of T./ormosa, also T. bella, which is the 

 next species described. The Cotteswold examples of T. tenuicosta were obtained in the 

 Gryphite-grit of Inferior Oolite at Rodborough Hill, associated with a multitude of 

 valves of Conchifera, including Trigonia sculpta, T./ormosa, T. P/iillipsii, and T. hemi- 

 sphcerica ; the two latter species very rarely. At the same locality, by passing upwards 

 some twenty feet, a hard shelly bed called Upper Trigonia-grit is attained, abounding in 

 fossils which are for the most part altogether distinct from those of the lower shelly bed ; 

 the Trigonia, which are also distinct, consist of the following species : T. costata (two 

 varieties), T. signata, T.producta, T. duplicata, T. angidata, T. V-costata, T. gemmata, 

 and T. denticulata. Both beds are, as a rule, destitute of Ammonites, excepting that the 

 upper bed has rarely been found to contain a specimen of A. Parkinsoni. The asso- 

 ciations of Trigonia here enumerated apply to beds of Inferior Oolite in the Cotteswold 

 Hills ; their dissimilarity to the Trigonia of the same formation in the Somersetshire and 

 Dorsetshire district is remarkable, more especially considering the small space by which 

 they are separated. 



A nearly allied and remarkable form of the Costata occurs in the rich fossiliferous 

 bed of Inferior Oolite in the vicinity of Bayeaux ; the general figure differs only slightly ; 

 it is apparently even shorter and more inflated ; the anteal truncation is somewhat 

 less decided. The most striking peculiarity consists in the presence of a minute row of 



