42 
Several Palaeontologists have proposed to divide the Rhyn - 
chonellce into genera, or sub-genera. In 1847, d’Orbigny 
(Paleontologie Francaise, Terrains Cretaees, Tome iv., page 
12,18 ±7,) gives the following subdivision of {)iq Rhychoneliidce 
Hemithiris , d’Orbigny ; Rhynchonella , Fischer ; Striyocephalus , 
Defrance; Rorambonites, Pander. D'Orbigny thus defines 
Hemithiris —“ Overture contigue a la charniere, point d’area, 
sommet entier.” 
D’Orbigny, (Pal. Franc., Cret., Tome iv, page 342, probably 
published later than 1847), again gives a definition of Hemi¬ 
thiris , and proposes to divide it into two genera, reserving the 
name Hemithiris for the species with fibrous texture, without 
pores or external spines; and Acanthothiris for species with 
spines “ Aux especes egalement de contextures fibreuses, mais 
pourvues d’epines perforees eparses ou par ligns, nous les nom- 
ruerons Acanthothiris. Nousy placons les Acanthothiris spinosa, 
d’Orb. et costata , d’Orb. de l’etage bajocien, et senticosa 
d’Orb. de l’etage oxforclien ; inscrites dans notre Prodrome sous 
le nom d ’Hemithiris.” 
McCoy (Paleozoic Fossils, 1855, page 199) divides the 
Rhynchonellidce into (1) Hemithiris , d’Orb. (2) Rhynchonella , 
Fischer; (3) Strigocephalus, Defrance; (4) Rentamerus, Sowerby; 
(5) Camarophoria, King. He thus describes the Grenus Hemithiris, 
“ General Character:—Foramen triangular not separated from the 
hinge,beak acute,pointed, entire,no cardinal area; entering valve 
with a small mesial septum ; apothysis short, arched, triangular, 
with small dental lamellce ; two strong diverging cardinal teeth 
bordering the opening in the larger valve, supported by dental 
lamellse extending to the surface of the valve (Type, Terebratula 
psittacea). “ The internal dental lamellae in the beak of receiving 
valve leave two diverging slits in the cast of the fossil shell.” 
Both Dr. Waagen and M. Oehlert, are in favour of retaining 
Acanthothyris as a distinct genus; and we are certainly inclined 
to divide the Jurassic Rhynchonellce into genera or sub-genera, 
Acanthothyris being one. 
The genus Rhynchonella has a long range in geological time, 
being found in nearly all formations from the Silurian upwards. 
Seven species are in existence at the present time. 
