Jura^ Neomniian and Chalk of Arkansas. — Marcou. 361 
The figures 11, 12, 18, 14, 15, 17 and 18 of plate v, and figs. 
20, 22, 23, 24, 26, 27 of plate vi, and figs. 28, 29, of plate vii 
represent very well the true and original Ostrea franldini 
of Owen and Coquand. Professor Hill, in his description, 
confounds the upper valve with the lower. The 0. franklini 
possesses only a very distant relation with the O. dubiensis., 
a much smaller and less pointed species described by 
Contejean and Etallon, but is truly related and allied to 
O. acuminata, by its pointed and acuminate lower valve. 
Then we have on plate vi, figs. 19 and 25, an oblique form 
of Ostrea, which belongs to the O. r(x,meri Quenstedt (Der 
Jura, tab. 77, fig. 22, p. 625); a species of the Argovian group 
of the Jura. 
And finally fig. 16 of plate v and more especially fig. 30, 
plate VII, are closely allied to a very common and character- 
istic form of the lower Oolite species of the Vesulian group, 
called Ostrea acuminata Sowerby, in the Jura mountains. 
At all events, all the forms figured by professor Hill are 
Jurassic forms, connected and allied with the Ostrea mrgula, 
O. rcdmeri and O. acuminata. 
Modiola, sp. ind. Hill, is a Mytilus allied to Mytilus 
longoivus Contejean of the Sequanian (Lower Kimmeridian) 
of the Jura {Lethea Bruntrutana, p. 224, plate xxix, fig. 9). 
Area g ratiota Hill. It is one of the largest J.rc'a of the 
Jurassic system. It resembles Area laufonensis Etallon 
{Lethea Bruntrutana, p. 215, plate xxvii, fig. 4 of the 
Sequanian of Laufon, Solenre, Switzerland). 
Area {Barhatia) parva missouriensis Hill, plate iv, fig. 5, 
is related to Area i'nmquivalvis of the Jura mountains. Mr. 
Hill refers "also probably" fig. 4a, 4b of the same plate to that 
species. Figure 4a belongs to a Nucula, and fig. 4b can not 
be referred to an Area. As to plate ii, fig. 22, also considered 
by professor Hill as belonging to Area parva missouriensis, 
it is a Nucula, allied to Nucula rostralis Lamk. of the upper 
Lias of Salius (Jura). 
Cyrena { Corhicula'/ ) arkansaensis. It is Corbiserenata 
Cont. or an extremely close species {Kimmeridian^ p. 55, 
plate xiii, fig. 10 and 11; and the Lethea Brttntrutana, t^. 
187, plate xxiii. fig. 2). Kimmeridian at Porrentruy and 
Montbelliard. 
Corhirula? {Astarief) pikensis Hill. It is an Astarte 
