164 GENESIS OF THE ARIETID. 
The superior lateral lobes are slightly serrated, and the superior lateral 
saddles have the generic division; but otherwise the lobes and saddles are 
apt to be entire. These characteristics were observed in one specimen on the 
fifth whorl. 
The general aspect of the shell is like that of the young of Psi. planorbe ; 
the pile, however, are.not merely broad prominent folds as in that species, 
but distinct immature pile, similar to those of other species of Arnioceras, and 
the form is quite distinct, besides being obscurely keeled. Some shells have 
straight pile and gibbous whorls, and others have bent pile and flatter whorls. 
Arnioceras Macdonelli, Hyrarr. 
Amm. Macdonelli, Portiock, Geol. Rep. Londonderry, p. 134, pl. xxix. a, fig. 12. 
Ariet. Macdonelli, Tarn and Buaxn, Yorkshire Lias, p. 290, pl. v. fig. 8 a-b. 
Ariet. nodotianus, Wrieur, Lias Amm., p. 300, pl. xxxvii. fig. 4. 
Wright's reproduction of Portlock’s figure and Portlock’s own figures show 
that this is a most remarkable modification of mserabile, occurring in the Rari- 
costatus bed, or what he first called the base of the Jamesoni bed. It is 
evidently an aged specimen, a very rare occurrence in this genus, and is con- 
sequently smooth. The young, however, have pile, and these and the section 
given by Portlock, the absence of channels, and compressed whorls, show it to 
be closely allied to nuserabile, var. aculidorsale. 
Arnioceras obtusiforme, Hyarr. 
Plate Il. Fig. 8-9a. Summ. PI, XII. Fig. 3. 
Amm. obliquecostatus, BRAuNS, Der Untere Jura, pl. i. fig. 38-5. 
Asteroceras obtusum (pars), Hyarr, Bull. Mus. Comp. Zool., I., No. 5, p. 79. 
Locality. —Semur. 
This species has the pils so closely resembling the curved depressed pile 
of Ast. obtusum, that it was formerly referred to that species. The young, 
however, are precisely similar to the young of Arnioceras, much too flat 
laterally for obfusum, and the pile never begin with tubercles or heavy folds, 
as in that species. The keel is depressed, and the channels are very. shallow or 
absent. 
Var. A. 
The pile are developed abruptly on the last quarter of the third whorl. 
The curved pil resemble those of the typical form of miserabile, var. cuneiforme. 
Var. B. 
The pilz are developed gradually, beginning with minute, regular folds on 
the first quarter of the third whorl, and they continue in the adult to resemble 
those of miserabile, var. cuneiforme, although the genicule become more promi- 
nent and make a nearer approach to those of Arn. semicostatum. The abdomen is 
narrow, the keel well defined, and in two specimens channels were faintly shown. | 
The whorls of varieties A and B are both more compressed than in variety C. 
