22 BRITISH FOSSIL TRIGONE. 



Trigonia perlata, Ag. Plate III, figs. 1, 2, 3. 



Trigonia clavellata, Young and Bird. Geol. Survey, 1828, pi. viii, fig. 18. 



— perlata, Agassiz. Trigonies, 18-10, p. 19. pi. iii, figs. 9 — 11. 



— — Hebert. Trigonies clavellees, Jour, de Conchyliologie, 186 1, pi. 



vii, fig. 2. 



T. perlata, Ag., has not unfrequently been mistaken for T. clavellata, Sow., with 

 which, indeed, it possesses some strong affinities. These errors are for the most part 

 to be referred to the very imperfect single figure of the adult form given by Agassiz, and 

 to his having mistaken the species of Sowerby, and figured another and very different 

 form for his T. clavellata ; fortunately M. Hebert has given a good figure and precise 

 description of T. perlata in the memoir above cited on some clavellated Trigonias of the 

 Oxfordian Rocks. Although the features which distinguish the T. clavellata of Weymouth 

 from the T. perlata of Pickering had long been present to my mind, it was the memoir 

 of Hebert that enabled me to identify the latter with the species of Agassiz. Adult 

 specimens of T. perlata agree in size with those of the other species. Young specimens 

 are smaller than those of T. clavellata, and have their ornamentation much more minute, 

 as exemplified in the rows of tuberculated costse, and in the tubercles upon the carinas ; 

 the form also is much more pointed and produced, both at the apex and the opposite 

 extremity of the valves, so that, even when comparing young specimens of both species, 

 their distinctness is evident. Adult specimens of T. perlata have much variability in the 

 number of costse and in the relative size of tubercles ; occasionally the costae form narrow 

 sub-tuberculatcd ridges, and the angle at which they approach the carina differs, but the 

 angle always exceeds that which occurs in T. clavellata. The apices are more produced, 

 narrow, and more distinctly recurved. After making allowance for occasional variability, 

 this latter feature is very persistent ; the opposite extremity is as constantly more 

 attenuated and even rostrated. This figure is produced, not by an actual difference in 

 the measurement across that part of the valve, but by the greater angle which that 

 portion of the area forms with the costated portion of the valve, so that when the valve is 

 viewed laterally, the posteal portion of the area is but little seen, and is not elevated as 

 in T. clavellata ; an appearance of greater breadth and roundness is thus imparted to the 

 posteal extremity of the latter species. The upper portion of the area is more depressed ; 

 but there is never any distinct furrow, and never any indications of a second row of 

 tubercles, as in T. clavellata. The three distinctive features, therefore, which are imme- 

 diately evident are the smaller and more pointed tubercles, the more narrow and recurved 

 apex, and the more produced and narrow posteal extremity in T. perlata. The more 

 distinctly ridged specimens have the interior borders of the valves scalloped. The valves 

 of Trigonia in the Coralline Oolite of Pickering occupy about a foot in thickness ; the 

 specimens are of all periods of growth, and the valves are invariably disunited. Their 



