RHYNCHONELLID, — 315 
plate deeply divided, supporting oral lamelle, rarely provided 
with spiral processes; muscular impressions grouped .-as_ in 
Terebratula; vascular impressions consisting of two principal 
trunks in each valve, narrow, dichotomizing, angular, the prin- 
cipal posterior branches inclosing ovarian spaces. 
Animal (of Rhynchonella) with elongated spiral arms, directed 
inwards, towards the concavity of the dorsal valve ; alimentary 
canal terminating behind the insertion of the adductor in the 
ventral valve; mantle not adhering, its margin fringed with a 
few short sete. 
RHAYNCHONELLA, Fischer, 1809. 
Syn.—Hypothyris, Phil., 1841. Hemithyris, d’Orbigny, 1847. 
Cyclothyris, M’Coy, 1844. Trigonella (part), Fischer, 1809 (not 
L. nor Dacosta). Stenochisma, Hall (part), 1847. Khyneotrema, 
Hall, 1860. 
Distr.—6 sp. R. psittacea, Chemn. (exxxvi, 69-71). Labrador 
(low-water?), Hudson’s Bay (100 fathoms), Melville Island, 
Sitka, Icey Sea. R&R. nigricans, Sby. (cxxxvi, 72). New Zealand, 
_19 fathoms. Fossil, 500 sp. Lower Silurian—; North and 
South America, Europe, Thibet, China. 2. vespertilio, d’Orb. 
(cxxxvi, 68). 
Shell trigonal, acutely beaked, usually plaited; dorsal valve 
elevated in front, depressed at the sides; ventral valve flattened, 
or hollowed along the centre, hinge-plates supporting two slender 
curved lamellz ; dental plates diverging. 
The foramen is at first only an angular notch in the hinge-line 
of the ventral valve, but the growth of the deltidium usually 
renders it complete in the adult shell; in the cretaceous species 
it is tubular. In R. acuminata (exxxvi, 73, 74), and many 
other paleozoiec examples, the beak is so closely incurved as to 
allow no space fora pedicel. Both the recent Rhynchonelle are 
black; &. octoplicata of the Chalk sometimes retains six dark 
spots. 
ACANTHOTHYRIS, d’Orb., 1850. Exterior surface spinous. &. 
spinosa, Schloth. (exxxvi, 75). Jurassic. 
RHYNCHOPORA, King, 1856. Valves having a punctate structure. 
R. Geinitziana, Vern. Dyas. 
LEIORHYNCHUS, Hall, 1860. Proposed for forms marked by 
plications on the mesial fold and sinus, and sometimes with 
obscure or distinct plications on the lateral portions of the shell. 
13 sp. Devonian; United States. 
EatontA, Hall, 1859. 
Etym.—Dedicated to the late Professor Amos Eaton. 
Syn.—Elonia, Meek and Worthen. 
Distr.—Fossil, 7 sp. Upper Silurian; United States. LZ. 
medialis, Hall (exxxvi, 76-79). 
