44 BRITISH FOSSIL TRIGONE. 



Trigonia spinulosa, Young and Bird. Plate III, figs. 4, 5, G. 



Trigonia spinulosa, Young and Bird. Geol. Survey, 1828, p. 225. 



— striata. Phil. Geol. York., 1829, 1, pi. xi, fig. 38 (not T. striata, Miller). 



— tuberculata, Agassiz. Trigonies, 1842, p. 20, t. ii, fig. 17, et tab. ix, 



figs. 6—8. 



— — Oppel. Juraformation, 1856, p. 407. 



— striata, jyOrbigmj. Prodrome, vol. i, 1850, p. 278, No. 312 (supposed by 



him to be young examples of T. striata). 



Shell ovately trigonal, moderately convex, umbones not prominent, much incurved, 

 obtuse but only slightly recurved, antero-mesial ; anterior side short, curved ellipticaliy, 

 with the lower border, posterior side, produced and compressed. Area moderately large, 

 transversely irregularly plicated, divided by a delicately tuberculated carina bordering a 

 furrow ; the two bounding carinas are large, with prominent rounded tubercles ; the superior 

 half of the area is more depressed than the other. The escutcheon is very large and 

 depressed, but its upper border is moderately raised throughout its length. The other 

 portion of the surface has about fifteen rows of large, raised, tuberculated costse, of which 

 the first formed seven are simply concentric ; all the rows are small, ridge-like, and only 

 subtuberculated anteally ; the tubercles in the rows are distinct, but slightly compressed 

 laterally, and increase in size posteally ; the general direction of the rows is nearly hori- 

 zontal anteally, or slightly directed downwards ; posteally they curve upwards rather sud- 

 denly and are united to the carina at a considerable angle ; they therefore most frequently 

 form a kind of slight undulation posteally at the part where the tubercles are the largest. 



The three tuberculated carinas and median groove are usually well defined, but occa- 

 sionally, as in the second specimen figured by Agassiz, there is no median row of 

 tubercles bordering the groove. 



No figure of this species is given in the work of Messrs. Young and Bird, but their 

 description is sufficiently precise and comprehensive to leave no doubt of the species 

 intended. The figure given in the ' Geology of Yorkshire' is very characteristic and must 

 have been named striata from mistaken recollection, as it is impossible that it could have 

 resulted from comparing the species of Miller with that from Blue Wyke ; the first figure 

 of T. tuberculata, Ag. ('Trig.,' t. ii, fig. 17), has been drawn from a specimen distorted by 

 compression. Specimens differ considerably in the proportions of their length and 

 height. 



D'Orbigny ('Prodrome,' i, p. 278) has fallen into the same mistake as Phillips in 

 regarding T. tuberculata as the young of T. striata; the very coarsely drawn and indif- 

 ferent figures in the ' Mineral Conchology' of T. striata will perhaps account for these errors. 

 T. spinulosa is more nearly allied to T. formosa, Lye, but comparison will show that 

 the former is more convex and much more produced posteally ; the figure of the rows of 

 costse is different, the greater size of the posteal tubercles and their separation in the 



