164 GENESIS OF THE ARIETIDiE. 



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 Psil. planorbe ; 

 the pila?, however, are not merely broad prominent folds as in that species, 

 but distinct immature pila3, similar to those of other species of Arnioceras, and 

 the form is quite distinct, besides being obscurely keeled. Some shells have 

 straight pilte and gibbous whorls, and others have bent pilse and flatter whorls. 



Arnioceras Macdonelli, Hyatt. 



Amm. Macdonelli, Portlock, Geol. Rep. Londonderry, p. 134, pi. xxix. a, fig. 12. 

 Ariet. Macdonelli, Tate and Blake, Yorkshire Lias, p. 290, pi. v. fig. 8 a-b. 

 Ariet. nodotianus, Wright, Lias Amm., p. 300, pi. xxxvii. fig. 4. 



Wright's reproduction of Portlock's figure and Portlock's own figures show 

 that this is a most remarkable modification of miserabile, 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 pilse, and these and the section 

 given by Portlock, the absence of channels, and compressed whorls, show it to 

 be closely allied to miserabile, var. acatidorsale. 



Arnioceras obtusiforme, Hyatt. 



Plate II. Fig. 8-9 a. Summ. PI. XII. Fig. 3. 



Amm. obliquecostatus, Brauns, Der Untere Jura, pi. i. fig. 3-5. 

 Asteroceras obtusum (pars), Hyatt, Bull. Mus. Comp. Zobl., I., No. 5, p. 79. 



Locality. — Semur. 



This species has the pike so closely resembling the curved depressed pike 

 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 obtusum, and the pilas 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 pike are developed abruptly on the last quarter of the third whorl. 

 The curved pike resemble those of the typical form of miserabile, var. cuneiforme. 



Var. B. 

 The pilae are developed gradually, beginning Avith 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 geniculse become more promi- 

 nent and make a nearer approach to those of Am. 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. 



