113 



depressed and plain, and is also enclosed by an inner 

 tuberculated ridge. There- are several species, many of which 

 prevail in the Middle and Upper Oolites of Great Britain. Eleven 

 species of this section have been met with in Dorsetshire. 



T. Voltzii, Agassiz., Kim. Clay. 



T. Pellati, Mun. Chalm., Kim. Clay. 



T. incurva, Benett, Kim. Clay. 



T. cymla, Contjean, Portland Sand. 



T. muricata, Lye, Portland Oolite. 



T. clavellata, Sow., Lower Calc. Grit. 



T. perlata, Agassiz., Lower Calc. Grit. 



T. irregular is, Seebach., Oxford Clay. 



T. striata, Muller, Inf. Oolite. 



T. formosa, Lye., Inf. Oolite. 



T. signata, Agass, Inf. Oolite. 



Dr. Lycett excludes T. fironnu, which had hitherto been 

 accepted as British, on the authority of M. Hebert, who con- 

 sidered a small fossil from the lower Calcareous Grit-beds of 

 Weymouth, to be T. Bronnii, but which, after careful examina- 

 tion, Dr. Lycett decides it to be merely a form of T. clavellata. 



The section Undulates differs from the Clavellata in the costee ; 

 which are undulated and not unfrequently broken into two dis- 

 tinct series of rows, of which the anteal are the smaller and 

 more numerous. Some, as T. conjungens, have ridges bearing 

 tubercles ; the area has a mesial furrow, and the escutcheon is 

 always plain. 



It has two Dorsetshire representatives, T. conjungens and T 

 literata, both from the Inf. Oolite in the neighbourhood of Brad- 

 ford Abbas. 



The section Glalra differs from the above in the slight differ- 

 ence of the area from the other portions of the valve, which 

 although fairly defined, is for the most part destitute of carineo 

 or only indications of them in the region of the umboiies. The 

 anteal portion of the valve has the costse moie or less prominent. 



Dorsetshire possesses five of the seven British species of this 

 section. 



