﻿SUPPLEMENT. 11 



imperfectly developed ; the few last cost« enlarge much posteally, their tubercles become 

 more prominent ; they approach the marginal carina at a considerable angle, the last three 

 or four rows terminating in varices, which are attenuated and not distinctly tuberculated ; 

 the larger specimen has about fifteen, the smaller one thirteen costee ; the lines of growth 

 are moderately distinct upon both specimens. 



Length of the larger specimen 29 lines ; height 14 lines. The smaller specimen, 

 which is more imperfect, has the length about 17 lines. 



The smallness, inequality, and irregularity of the tubercles, together vdth the 

 tendency to effacement about the middle of the valve, together with the enlargement of 

 the costee posteally, are the most prominent distinguishing features of this small variety, 

 which appears to be rare, as Mr. Walford can only refer to three specimens. 



Position and locality. — Hook Norton in the lower beds, limestone with courses of 

 sand, B and C of Mr. Walford's section (page 7). 



Tbigonia pulchella, Aff. Trigon. Monog., Plate XXXVIII, figs. 10 — 12 ; andTrigon. 



Supplement, Plate III, figs. 7 — 12. 



The figures of this pretty little Liassic species (given Plate XXXVIII, figs. 10 — 12, 

 page 185) are of small dimensions, and do not illustrate the shell in its more advanced 

 condition of growth ; I therefore gladly take the present opportunity to figure other and 

 more suitable specimens; The two larger shells (PI. Ill, figs. 10 — 12) exemplify the 

 ultimate stage of growth and exhibit a considerable change in the ornamentation of 

 the large area; it will be observed that the acute transverse costoB, which in 

 immature forms are widely separated, become ultimately closely arranged and even 

 crowded near to the siphonal border. 



Specimens in my collection, showing the interiors of the valves, have three alternating 

 prominences and pits near to the posteal extremity of the pallial border. It would appear 

 that the specimens at the disposal of Agassiz did not exhibit the interiors of the valves, or 

 no doubt he would gladly have figured and described these internal features; his 

 specimens arc only of median size ; the larger single specimen figured by Quenstedt 

 (' Der Jura,' plate xliv, fig. 1) was probably imperfect posteally, and so obtained the 

 short truncated aspect which the species does not possess. 



Position and Locality. — Triyonia pulchella has been collected at Lincoln, in the 

 Upper Lias in the Zone of Ammonites communis, both by Mr. Keeping, of Cambridge, 

 myself, and Mr. W. D. Carr ; to the last I am indebted for the unusually fine specimens 

 herewith depicted, I am not aware that the species has been obtained at any other 

 British locality. 



