6 BRITISH FOSSIL TRIGONLE. 



Sow., T. perlata, Ag., T. corattina, D'Orb., T. Bupellensis, D'Orb., T. impressa, Sow., 

 T. signata, Ag., T. spinulosa, Y. & B., T. striata, Miller, T. Bronnii, Ag., T. irregularis, 

 Seebach, T. triquetra, Seeb., T. Pellati, Mun.-Chal., T. P/dllipsi, Mor. and Lye, T. 

 Moretoni, Mor. and Lye, T. imbricata, Sow., T. muricata, Goldf., T. parcinoda, Lye, 

 T. Woodwardi, Lye, T. radiata, Benett, T. Ramsayi, Wright, T. incurva, Benett, T. 

 complanata, Lye, T. Scarburgensis, Lye, T. Juddiana, Lye, T. formosa, Lye, T. tuber- 

 culosa, Lye, T. Griesbachi, Lye, T. ingens, Lye Of these, the only species that occurs 

 in the Cretaceous Rocks is T. ingens. 



3rd. — Undulate. 



This section was constituted by Agassiz to receive species whose general form 

 approximates to the Clavellata, but whose costse, whether with or without distinct 

 tubercles, have an undulation or an angle towards the middle or the posteal portions of 

 the costse ; it also not unfrequently happens that the costse are broken into two distinct 

 series of rows, of which the anteal series are the smaller and more numerous ; the rows are 

 usually ridge-like, sometimes nearly plain, or in other species with a few tubercles or 

 varices ; the area is narrow and the bounding carinse inconspicuous ; there is also a mesial 

 furrow, bordering which a line of small tubercles may occasionally be traced in immature 

 forms ; the escutcheon is always plain. The boundaries between this section and the 

 ClavellatcB are by no means clearly defined, as some species of the latter have not 

 uncommonly a kind of angle or undulation in their costse either mesially or posteally ; this 

 feature, therefore, appears more appropriate to a species than to a section, and it is 

 not probably of any further value than as a convenience in the arrangement of 

 the species. Like the Clavellala, it is a Jurassic section. It has afforded nineteen 

 British species, as follows : — T. angulata, Sow., T. litter ata, Young and Bird, T. Clytia, 

 D'Orbigny, T. tripartita, Forbes, T. conjungens, Phillips, T. Carrei, Mun.-Chal., 

 T. subglobosa, Mor. and Lye, T. LecTcenbyi, Lye, T. v-costata, Lye, T. producta, Lye, 

 T. SJiarpiana, Lye, T. compta, Lye, T. Painei, Lye, T. paucicosta, Lye, T.fiecta, Lye, 

 T. minor, Lye, T. costatula, Lye, T. composita, Lye, T. arata, Lye 



4th. — Glabrae. 



The Glabra or Laves was a section founded by Agassiz upon very insufficient 

 materials, which led to errors in his definition of the section ; he described it as without 

 ornamentation, without tubercles or costae, and resembling a large Unio. With some 

 modification in the sectional characters it will be found to constitute a group sufficiently 

 distinctive, not, indeed, devoid of ornamentation, for this appears to be a feature essential 

 to the entire genus. The usual figure, as remarked by Agassiz, is somewhat inflated and 



