Davidson—fecent and Tertiary Species of Thecidium. 19 
A glance at the carefully enlarged illustrations in Pl. I, will, I trust, 
bear out the view here taken. This Thecidium does not appear to 
be rare in the Lower Oligocene beds of Latdorf, near Bernburg, in 
North Germany. As far as we at present know, this would be the 
oldest type or representative of the Mediterranean species. 
2. THECIDIUM MEDITERRANEUM, var. TEsTUDINARIUM, Michelotti, sp. 
Pl. I., figs. 4 & 5; Pl, IL, fig. 4. 
Thecidea testudinaria, Michelotti: Annali delle Scienze del Regno Lombardo- 
Veneto, 1840; and Actes de l’Académie Hollandaise des Sciences, 2e sér. 
vol. i., 1847, p. 79, pl. 2, fig. 26. 
This Thecidium, by its external shape and interior characters, 
appears to bear so much resemblance to the Mediterranean shell that 
I doubt much whether we should be justified in considering it more 
than a variety. Its beak and area are at times considerably deve- 
loped ; but such is sometimes found to be the case with the recent 
type. In allessential points the interior resembles that of Theeidium 
Mediterraneum. A small and the only difference I am able to detect 
consists in the central (tongue-shaped) ‘ascending process’ being a 
little narrower, and commencing a little lower down, or nearer the 
front, than is usually seen in the recent shell. This Thecidium does 
not appear to be rare in the Middle Miocene beds near Turin; but 
it appears to be difficult to obtain perfect examples showing the 
interior of the dorsal valve. 
3. THecipium Apamsi, Macdonald [variety of Th, Mediterraneum ?]. 
Pl. I., figs. 10-13. 
Thecideum Adamst, Macdonald; Quart. Journ. Geol. Soc., vol. xix. p. 517, 1863. 
This shell does not appear to have attained the dimensions of the 
preceding two varieties of Th. Mediterraneum. By its general shape 
and interior arrangements, it so closely resembles the recent Medi- 
terranean form that I feel doubtful whether it should be considered 
more than as a small race or variety of Thecidium Mediterraneum. 
Its exterior shape and characters have been correctly described by 
Mr. Macdonald, who does not fail to observe ‘that its nearest ally is 
the recent Theceidium Mediterraneum.’ Thecidium Adamsi has often, 
although not always, moulded a portion of the back of its larger 
valve upon slender stems of some concentrically ribbed coral, which 
has been reproduced in relief, with all the original markings, on the 
flattened dorsal valve (Pl. I., fig. 10, a, ,) ; but when the shell has been 
fixed to some flat or rugose marine object by a larger or smaller 
portion of the extremity of its beak, the dorsal valve exactly resem- 
* bles that of the recent Thecidium Mediterraneum (P1. I. fig. 11). The 
interior of the dorsal valve has not been quite correctly described 
or figured in the Journal of the Geological Society ; I have therefore 
drawn this valve with great care from some perfect examples sent 
to me by Dr. Adams, of Malta ; and these figures, I trust, will con- 
vey to the reader’s mind the intimate resemblance this shell bears to 
the Mediterranean species ; so much so indeed is this the case that 
c 2 
