RHY NCHONELLTD^. 315 



plate deepl}'- divided, supporting oral lamellos, rarel}^ 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 i)osterior 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 setae. 



Rhynchonella, Fischer, 1809. 



Si/n. — Hypothyris, Phil,, 1841. Hemithyris, d'Orbigny, 184Y. 

 Cyclothyris, M'Coy, 1844. Trigonella (part), Fischer, 1809 (not 

 L. nor Dacosta). Stenochisma, Hall (part), 1847. Rhyncotrema, 

 Hall, 1860. 



Distr. — 6 sp. R. 2:)sUtacea, Chemn. (cxxxvi, 69-71). Labrador 

 (low-water?), Hudson's Bay (100 fathoms), Melville Island, 

 Sitka, Icy Sea. B. nigricans^ Sby. (cxxxvi, 72). New Zealand, 

 19 fathoms. Fossil, 500 sp. Lower Silurian — ; North and 

 South America, Europe, Thibet, China. R. 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 lamellae ; dental plates diverging. 



The foramen is at first on\j 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 (cxxxvi, 73, 74), and many 

 other palaeozoic examples, the beak is so closely incurved as to 

 allow no space for a pedicel. Both the recent Rh3mchonellae are 

 black ; R. octoplicata of the Chalk sometimes retains six dark 

 spots, 



ACANTHOTHYRTS, d'Orb., 1850. Exterior surface spinous. R. 

 spinosa^ Schloth. (cxxxvi, 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. 



Eatonia, Hall, 1859. 

 Etym. — Dedicated to the late Professor Amos Eaton, 

 Syn. — Elonia, Meek and Worthen, 



Distr. — Fossil, 7 sp. Upper Silurian ; United States. E. 

 medialis^ Hall (cxxxvi, 76-79). 



