CO^'CLUDE^'G SYXOPTICAL OBSERVATIOXS. 221 



separate it into two specie?, the one with the few and large costse constituting his T. 

 sulcata (■' Trigonis,' p. 44). Possibly T. peninsulans may be equally variable, but un- 

 fortunately Coquand has given only a single figure and of the left valve only (' Terr. 

 Aptien d'Espagn.,' pi. sxiii, fig. 3). 



However clearly separated are the Cosfafa as a section, they also approach a very 

 difPerent section i^that of the GlahrtB) in certain Swabian species which have the middle 

 portion of the valve entirely destitute of surface ornaments, as in T. zonata, Ag., T. infer- 

 lavigafa, Quenst., and T. triangularis, Goldf. They also approach to the ClaveUafa in 

 T. hyhrida, Roem.. and in T. geographica, Ag., and in two less known forms alluded to at 

 p. 161 under the names of T. fimhriata^ Lye, and T. granigera. Cent., which have the 

 longitudinal costae and also the phcations upon the marginal carina minutely clavellated. 



The ClaveUata are peculiarly varied in species and locally abundant in the Middle 

 and Upper Oolitic Rocks of Britain, numbermg upwards of thirty-three species ; probably 

 they are not less abundant in France, Switzerland, and Germany, where, in the 

 Oxfordian and Kimmeridgian stages, they appear to attain their maximum of numbers, 

 and then suddenly disappear. 



The Undulata are exclusively .Jurassic, and little less varied than the ClaveUata. 

 Britain has twenty species. They occur in the Upper Lias, in the Supra-liassic 

 Sands, and more abundantly in the Lower Oolites ; locally they are gregarious. 

 In the Middle and Upper Oolites they are much less conspicuous, and are represented 

 by four species onlv. T. paucicosta alone can be said to be even locally abundant. 

 They disappear altogether in the lower portion of the Portland formation. 



The Scaphoidea, comparatively a small section, occurs only locally, and of few species, 

 chiefly in the Upper Lias and Inferior Oolite, after which nothing more is known of the 

 section until it reappears in the Lower Calcareous Grit of Yorkshire, and, after another 

 stratigraphical interval, widely separated, in the Middle Xeocomian beds of Norfolk, 

 where it is represented by two species, one of which, T. exaltata, nob., Plate XXXVIII, 

 is of gigantic dimensions. In the higher assemblages of Cretaceous fossils the 

 Scaphoidea are unknown. 



The Glabra in the circumstances under which they occur offer a marked contrast to 



the leading sectional forms of the genus both Jurassic and Cretaceous ; belonging to the 



whole of the Mesozoic Epoch, it is only in the Porilandian beds of Britain that they 



become predominating forms ; their occurrence as a single species dates from the earliest 



record of the senus in the Middle Lias above referred to. In the Inferior Oolite thev 



are represented by the rare, anomalous, and in some respects almost unique, T. 



Beeslegana, after which nothing more is known of the section until its reappcdirance in 



the Portland formation, represented in Britain by five species, two of which, T. gi66osa 



and T. Damoniana, occur in great abimdance. All of these Portlandian Glabree have a 



considerable family resemblance in their short, subglobose, or ovately rounded forms ; 



their surface ornaments consisting for the most part of small, longitudinal, subtuber- 



29 



