Davidson-——Recent and Tertiary Species of Thecidium. 13 
Thecidea by Defrance* in 1821, and corrected into Thecidium 
by George Sowerby f in 1823. 
The shells of this genus are all small, none of the many 
species at present known being more than three-fourths of an 
inch either in length or breadth. Their shape is very varied, 
and they have existed from the Triassic period up to the 
present time. A single species only has been described from 
the Trias. The Jurassic forms are numerous, and have been 
fully described and illustrated by M. E. E. Deslongchamps,t 
Mr. C. Moore,|| and myself.§ The Cretaceous forms have 
likewise been admirably described and figured by M. Bosquet,4 
Professor Suess,** and others; but the Tertiary forms have not 
hitherto received that full attention they deserve. The recent 
Thecidium Mediterraneum has been figured by several natu- 
ralists; and it is to M. Lacaze Duthiers that we are indebted 
for the most complete description of the animal we at present 
possess.tf It may, however, be as well to observe that nine 
years prior to M. Duthiers’ researches, Mr. 8. P. Woodward 
and myself had published an enlarged illustration of the in- 
terior of the dorsal valve, accompanied by the following 
explanation: ‘ Thecidium has a calcareous loop, folded into two 
or more lobes, lying in hollows of corresponding form, exca- 
vated in the substance of the smaller valve. This loop, or 
apophysary ridge, supports the brachial membrane, whose thick- 
ened and ciliated (cirrated) margin is apparently attached to 
the inner sides of the grooves. ‘The cilia (cirri) are very long, 
especially the outer series, which are directed inwards in the 
dried specimens.’ t{ We were thus moreover enabled to show 
that D’Orbigny’s supposition that Thecidium was unprovided 
with ‘ oral arms’ was erroneous, and that consequently his term 
‘ Abrachiopodes’ could not be admitted. 
The interior arrangements of the smaller or dorsal valve 
* Defr. in Fér. Tabl. Syst., 38, 1821. See also De Blainville, Manual Malac., 
516, 629, 1825; Dict. Sc. Nat. liii., 1828; and Risso, Europe Mérid., 393, 1826. 
+ Genera of Recent and Fossil Shells. 1820-24. : 
t Mémoires de la Société Linnéenne de Normandie, vol. ix., 1853, and vol. x., 
1855. 
|| Proceedings of the Somersetshire Archeological and Natural History Society, 
1854. See also ‘ Geologist,’ vol. iv. p. 198, pl. 1. 1861. 
§ A Monograph of British Jurassic Brachiopoda, 1851. 
“| Monographie des Brachiopodes fossiles du Terrain Crétacé supérieur du Duché 
de Limbourg, 1859. 
** Ueber die Brachial-Vorrichtung bei Thecidium, 1853 ; Sitzungsber. Akademie 
der Wissenschaften, vol. xi. p. 991. 
++ Histoire Naturelle des Brachiopodes vivants de la Mediterranée, Annales des 
Sciences Naturelles, 4e Sec. Zool., vol. xv., 1861. 
{+ Annals and Mag. Nat. Hist., 2nd series, vol. ix., May, 1852, p. 374; and 
Manual of the Mollusca, by 8. P. Woodward, p. 221, 1856. 
