94 SUPPLEMENT TO THE 



The largest specimen of Betzia Salteri that has passed through my hands measured — 

 length 6, breadth 8, depth 4 lines. 



Betzia Boucliardi, Dav., is generally a much smaller shell with an elongated oval 

 shape, and is far more abundant than B. Salteri. One specimen, however, of B. 

 Bouchardi attained 7 lines in length by C^ in breadth. B. Salteri and B. Boucliardi 

 are therefore well characterised and distinct species. 



Genus — Meristina, Suess, 1851. 



5. Meristina didyma, Dalman, sp. Dav., Sil. Mon., PI. XII, figs. 1 — 10 ; and Sil. Sup., 



PI. IV, figs. 20—23. 



Tekebbatula didyma, Dalman. K. Vet.-Akad. Handl., pi. vi, fig. 7, 1828. 

 Mebistella didyma, Bav. Sil. Mon., p. 112, pi. xxi, figs. 1 — 10, 1867. 

 Meristina nitida, Hall. Pal. New York, vol. iv, p. 299, 1867. 



At p. 113 of my ' Silurian Monograph' I described the exterior of this species, but all I 

 knew at the time of its interior character was that it possessed spiral appendages for the 

 support of the labial appendages. The spirals are somewhat oval in shape, and each 

 contains about nine convolutions. The two principal stems of the spiral coils are attached 

 to the hinge-plate, and, after extending a little way into the interior of the shell between 

 the spirals, are suddenly bent backwards and towards the hinge. After forming a curve 

 they converge to about half the length of the spiral, giving forth at that place short 

 converging lamellae, which are slightly or more strongly undulated in shape, and which 

 proceed straight across between the spirals to their ventral side before uniting in a sharp 

 angular point. This point is turned upwards more or less gently or abruptly, almost imme- 

 diately behind the primary lamellae on the ventral side. The end of the loop wdiich is thus 

 turned upwards varies in length in different specimens, never being very long, and 

 sometimes barely perceptible. What was said of Betzia Salteri may also be said of 

 Meristina didyma, namely, that in some exceptional cases the loop differs from the 

 description here given in its having an upward slope instead of proceeding straight 

 across. The two primary lamellas giving off' the processes which form the loops diverge 

 again as they proceed towards the front, and by a gentle curve form the first spiral coil. 



Obs. — At p. 299 of the fourth volume of the ' Palaeontology of New York,' Prof. 

 Hall proposed a new genus, Meristina, for shells in which " the lamellm of spires are 

 united hy a simple looj)^ and mentions Meristina Maria and Meristina nitida, Hall, as 

 types of his genus. 



The beautiful preparations on which this genus was founded were made by Mr. 

 R. P. Whitfield for Prof. Hall's great work on the 'Palaeontology of New York.' 



