Hyatt.] 26 [May 20, 



Deroceras ziphoides. 



Amm. ziphoides Quenst., Der Jura, p. 130, pi. 15, fig. 11. 



This is really only a form of ziphius in the Lias, which has an 

 accelerated mode of development, and has" partly skipped the plani- 

 costan character of the abdomen. The pila? still cross the abdomen, 

 but have lost their broad planicostan aspect. 



Deroceras planicostum. 



Sowerby's specimens are mixed with latcecosta and Dudressieri. 

 These hardly afford the means of determining whether planicosta 

 deserves a separate name- from Dudressieri, but after a careful exam- 

 ination I doubt whether the form of planicosta can be separated from 

 the young of Dudressieri. It will be observed that planicosta is a 

 small species, and in many undoubtedly planicostan varieties the 

 characteristic spines of Dudressieri are assumed after the specimen 

 attains an unusually large size, so that it becomes impossible to sep- 

 arate them from the young of Dudressieri. Several of Sowerby's 

 specimens are unquestionably forms of this species. 



There are, however, some extreme forms of planicosta laterally 

 very flat and very narrow on the abdomen, for which it may be 

 found convenient to reserve a separate appellation. 



My remarks with regard to the affinity of this species with species 

 of the Arietidse should be more definite. They can apply only to 

 certain parallel or reversionary characteristics which are common to 

 both Arietidae and Microceras, and not attributable to any direct 

 genetic connection. 



Deroceras confusum. 



This should be Ammonites Lohbergensis Emerson. Deroceras confu- 

 sum Quenst. is very distinct. The figures of Quenst. are not ex- 

 actly correct. Fig. 8. pi. 72 of "Der Jura" has a hardly perceptible 

 keel connecting some of the abdominal ridges of the original speci- 

 men, but absent between others. The whorl is quite round in the 

 young, then acquires the form given in Fig. 8, and then that delin- 

 eated in Fig. 10. There are, however, still very faint signs of a keel 

 which is entirely lost between the oldest ridges. 



Amm. subplanicosta Oppel. 



This remarkable form, as seen in the Munich Collection, has young like biferum, 

 and in other respects resembles that species, but begins to acquire the planicostan 

 or latascostan pilae at a very early age, and in some specimens probably remains 

 similar to latcecosta throughout life. There can be but little doubt that it is a latae- 

 costan-like variety of M. biferum. 



