WENLOCK GROUP. 149 
WENLOCK CONTINUED. 
LAMELLIBRANCHIATA (Conchifera of Authors). 
Bivalve shells proper: differ from Brachiopods not only in the want of the spirally’ coiled and ciliated 
arms—the tentacles, four in number, being simple and flaccid—but in wanting the ventral shelly cover to the 
foot, which in the Brachiopod becomes the ventral valve. The foot on the contrary, though often giving birth 
to a byssus, is a free organ, useful for locomotion, and capable of great and varied evolutions. The valves 
in the infant state are open (as Rathke has shewn in Anodon), and the heart and intestine double (the 
heart is permanently double in the Brachiopod), and these coalesce as the animal grows and closes the 
valves. The muscles instead of all being directed to the foot and its appendages as in the TZerebratula, are 
chiefly used for transverse action to close the valves, a portion only beimg directed to the important foot. 
The higher genera, not requiring to be anchored (Cardiwm, Venus, &e.), do not spin a byssus. This, which 
arises from a large gland in the back of the foot, is represented by the anchor or plug of the Terebratula ; 
and, as I have lately shewn (Camb. Philos. Trans.), the operculum of the Gasteropod is an analogous organ. The 
Bivalves then stand half way between the snails and whelks, with free motion, and the permanently fixed 
and helpless Terebratula. 
It is important to observe, and Professor Phillips was the first to point it out (Mem. Geol. Surv. Vol. 1. 
Pt. 1, p. 264), that the lower genera only of Lamellibranchs—Arca and its allies, Avicula, Pterinea, Modiola, 
Mytilus, to which may be added Nucula, the freest of this group—are present in the earliest rocks in which 
fossils are found. To these may be added Conocardium, which, like Teredo and Pholadidea in our seas, have 
the valves soldered. 









Cc ad 9 
Garin of Reference eo McCov’s Names and References; Observations, &c. Numbers and Localities. 
Drawers.| Synopsis: and Figures of Genera. . 
FC Pl. 1 1, figs. 11—15, p. 258. | Avicula Danbyi, McCoy (Siluria, 2nd ed. Foss. | a. 803, Dudley, F. C.* 
59, figs. 2, 3). A common shell in the Lud- 
low Sandstones of Kendal. 
FC Avicula mira, Barr. MS. (Siluria, 2nd ed. | a. 820, a. 821, a. 822, left 
p. 253). valve; a. 823, right valve; 
a. 824, both valves united, 
rare, Dudley, F.C. 
FC p. 263. Pterinea Sowerbyi, McCoy (Siluria, 2nd ed. | a.818, right valve, Dudley, 
pl. 23, fig. 15. Avicula reticulata, Sil. Syst. | F.C. 
t. 6, fig. 3). A long direct shell, not trans- 
verse as In many species. 
Gda7 PI. 1 1, fig; 5; p. 259. Pterinea asperula, McCoy (Siluria, 2nd ed. | b. 720, Builth, W. Shale. 
p- 253, Foss. 59, fig. 4). 
Gd7 Pterinea lineatula, D’Orb. (Siluria, 2nd ed. pl. | b. 727, Myddleton Park, 
Ge5 23, fig. 16 Avicula lineata, Sow., is not the | Caermarthenshire (Wen- 
Avicula lineata of Goldfuss). lock Shale); b. 728, Dud- 
ley, F.C. 

1 F.C. Fletcher Collection. 
