220 BRITISH rOSSIL TRIGONI.E. 



In the Upper Lias and Supra-liassic Sands of Britain the usual Jurassic sectional 

 forms of Tnqonia, consisting of the Cosfafcc, ClavcUatcc, and Uiidiilafa, already acquired 

 some of the importance and variety by which they were subsequently characterised. 



In the Costata are surface ornaments and some oiher external features observed in the 

 'MyophoricB, but the umbones are never directed forwards as in that genus, and the hinge- 

 processes present differences, both in their figures and in their deeply sculptured sulca- 

 tions, which are not limited to one side only of each process ; the right valve presents the 

 more important differences, the marginal carina is not divided into two or three costellae 

 as in ^Iiiophoria, but forms a single carina larger than that of the left valve ; the costellae 

 upon the area are also fewer and larger than those of the other valve. The Lower Oolitic 

 rocks abound with this important sectional form, which in Britain is represented by 

 upwards of twelve species besides varieties ; abundant and dwarfed in the oolitic lime- 

 stones of the Inferior and Great Oolite, they acquire large dimensions in the clays and 

 argillaceous shales. The stratigraphical range of the section Costafa is not con- 

 siderable, ten of the British species occur only in the Lower Oolites ; one of these, 

 T. hemisphcerica, and its variety named gregaria. Plates XXXI and XXXIII, although 

 distinct generically from Myophoria, yet has some external resemblance to Myophoria 

 lineata, Munst., and to M. postera, Quenst. ; the variable and minute longitudinal costap 

 are more especially analogous. The British LTpper Oolites have only two additional 

 species of the Costatce special to those stages. These are T. Meriani, Ag., and T. monilifera, 

 Ag., so that we have no example higher than the Kimmeridge Clay ; and even T. monili- 

 fera (Plate XXXI), locally so abundant in the lower beds of that stage, is but a continua- 

 tion of the same form which occurs more rarely in the beds of Upper Calcareous 

 Grit. The Costata occur apparently in greater variety in the Upper Jurassic rocks of 

 Prance, Germany, and Switzerland. Only a minority of these have been figured and 

 described : probably more than twice the number of ascertained species w^ould be required 

 to make up the sum of the European Costatce. All of them disappear with the lower 

 portion of the Portland formation. 



Two abnormal costated forms occur in the Neocomian period if, indeed, we should 

 include with that section T. carinata, Ag., and T. peninsidaris, Coq., species which were 

 deprived of the surface ornaments ere they attained adult growth, when one, and perhaps 

 each of them, acquired a byssal aperture. T. carinata is always recognised by the great 

 obliquity of the costae ; T. peninsularis has the costae very irregular, imperfect in the rows, 

 subangulated, and almost evanescent. In the distinctions between the opposite valves of 

 the CoiYfl^^ described at p, 9, we see reproduced, modified, and less strongly defined some 

 of the generic features of the Myophoria fading out, it may be, and modified upon the 

 TrigonicB. The variability in the number and size of the longitudinal costae described as 

 marking varieties of species in this Monograph were also produced as a variable feature 

 in the more ancient Myophoria postera, of the Upper Trias ; it is equally present in the 

 Neocomian T. carinata, where the variability is so considerable as to induce Agassiz to 



