﻿GLABRAE. 



99 



The hinge dental processes are exposed in two of the specimens figured ; they agree 

 with those of the Glabra generally, and are less massive than in those of other Jurassic 

 sectional forms. The internal mould has the adductor scars well developed ; there are 

 also traces of the external encircling rugae ; the test appears to be thin. The external 

 rugae differ much in their prominence in different specimens, so that in some instances 

 the general surface is smooth, and has rugae only near to the pedal border. 



M. Dumortier described T. Lingonensis from the Marlstone beds of the valley of the 

 Rhone, near to Langres ; his figure has a slight depression of the surface anteal to the angle 

 of the valve. None of the British specimens possess this feature, which probably, there- 

 fore, is only accidental. 



Mr. R. Tate, now a resident at Redcar, to whom we owe its discovery as a British 

 species, states that it occurs in the main seam of ironstone throughout the Cleveland 

 district in the zone of Ammonites spinatus, and that it has been obtained at the following 

 localities : 



Skinningrone mines ; Hobb Hill mine, near Saltburn ; Eston mines, near Middles- 

 boro' ; Belman Bank and Challoner mines, near Guisborough ; that the species is rare, 

 excepting at Eston, but well-preserved specimens are everywhere rare. The two larger 

 of our specimens were obtained by Mr. G. Lee, manager of the Eston mines at that 

 locality, and generously presented by him for the present Monograph. 



Examples of this remarkable Trigonia are in the museum of the Royal School of 

 Mines ; in the museum of the Yorkshire Philosophical Society ; in the collection of Mr. 

 R,. Tate at Redcar ; in that of Mr. G. Lee at Eston ; and in my own cabinet — all from 

 the ironstone of the Cleveland district. The British Museum has a specimen obtained 

 by the late Miss Baker, of Northampton, in the Middle Lias at Preston Capes in that 

 county ; no other example from the Lias of the midland or southern counties has come 

 under my observation. 



Affinities. — The almost entire absence of ornaments upon the surface associates it 

 with the Glabra, a section which has only a small number of ascertained species, and, 

 unlike other sections of the genus, is not limited to one portion of the Mesozoic period, 

 but occurs at intervals widely separated stratigraphically ; thus, the Clavellata, the 

 Undulates, and the Costata, are limited almost exclusively to the Jurassic formations. 

 The Quadrata and the Scabra are not less strictly Cretaceous forms, but the Glabra, 

 although represented by few species, constitute a section which embraces nearly the 

 entire limits of the Mesozoic period. 



T. Lingonensis, the oldest known Trigonia, is limited to the Middle Lias. The next 

 known example of the section is our T. Beesleyana, which occurs not less rarely in the 

 Inferior Oolite at a single locality. From that position the section appears to be absent 

 until we arrive at the Portland formation, where it becomes the predominating section of 

 the genus, and is represented in Britain by five species, two of which are abundant. 

 Again, after a long stratigraphical interval, the section reappears in the middle portion of 



