72 BULLETIN OF THE 



The materials in the possession of the Museum afford ample means 

 for the successful completion of such a plan, which, more than any 

 other, demands large numbers of specimens. They consist of collec- 

 tions purchased from Professors Bronn and L. de Koninck, MM. Bou- 

 cault and Duval, Dr. A. Krantz, and others, besides those obtained by 

 exchange, among which is a valuable collection, numbering many 

 species, from the Museum of Stuttgart. 



I am indebted to Professor L. Agassiz for the free use of all the 

 specimens in these collections, and desire to express here my acknowl- 

 edgment of the facilities for study given me both by himself and the 

 Institution. 



The position of the female Argonauta in its shelly ease, and of the 

 Nautilus in its shell, show conclusively that the periphery of the whorls 

 of an Ammonite is the abdominal side, as stated by Richard Owen and 

 Pictet. This view, therefore, has been adopted, and the outer side of 

 the whorl is called " the abdominal," and the inner " the dorsal side.*' 

 in accordance with their opinion. 



No further changes have been made in the nomenclature generally 

 employetl, with the exception of the use of the words " pike " for ribs, 

 and "genicular" for the knees of the ribs, these being found somewhat 

 more convenient in the description of the species than the ordinary 

 terms. 



Loivcr Lias. 



PSILOCERATID^3. 



Shell smooth. Umbilicus open, exposing the sides of the whorls; sides 

 depressed. 



Psiloceras Hyatt. 



Abdomen smooth; shell often folded ; sides depressed ; septa foliated. 

 "Win iris enveloped to the line of the superior lateral lobes. 



Psiloceras psilonotum Hyatt. 



Ammonites psilonotus Quens'dt, Die Cepli., p. '3, pi. 3, fig. 18. 



Loc. Nellingen, Balingen, Rudern, and Semur; Coll. L. de Koninck, 

 Prof. Fraas, Mus. of Stuttgart, L. Agassiz, and Boucault. 



It is quite probable that Annii. erugatus Bean (Phil. Geol. York, p. 1C8, 

 pi. 13, figs. 1-3) is identical with this species, and if so, it will become 

 necessary to changt the name to Psil. erugatus, and consider Pail, pailono- 

 tus as a synonyme. 



