2'2i> PSILOCERAS, ARNIOCERAS, OPHTOCERAS. 



periphery of the shell, he calls this outer side of the latter "ab- 

 dominal," and the inner or sutural side " dorsal." In quoting 

 his own diagnoses I have followed him, but regard this reversal 

 of terms as objectionable, inasmuch as their exceptional use in 

 the shells of tetrabranchiates must give rise to a great confusion. 

 He uses also the word " pilae " for ribs, and " geniculse " for the 

 knees of the ribs. 



Prof. Hyatt has, subsequently to the publication of his classi- 

 fication (as quoted below), changed his views somewhat as to 

 portions thereof (Bost. Proc., xvi-xviii). He has adopted such 

 extreme " development " views upon the subject, as render his 

 later groups difficult to define. A clear exposition of the re- 

 versed position of the animal of the tetrabranchiate, in relation 

 to its shell, may be found in a paper by Prof. Owen, Zool. Proc., 

 955, 1818. 



Family PSILOCERATID.E. Shell smooth, umbilicus open, 

 exposing the sides of the whorls ; sides depressed. 



PSILOCERAS. Abdomen smooth ; shell often folded ; sides de- 

 pressed ; septa foliated ; whorls enveloped to the line of the 



superior lateral lobes. 



Lower Lias. 

 P. PSILONOTUM, Quenst. T. 106, figs. 603, 604. 



Family DISCOCERATID^E. 



ARNIOCERAS. Abdomen keeled and channeled, but both parts 

 are variable, being sharply defined in some species, and very 

 shallow in others. Abdominal lobe shallow and broad ; not so 

 deep as the superior lateral lobe ; deeper than the inferior lateral ; 

 both divided equally. Superior lateral cell equally divided. 

 The young retain the smooth character for some time during 

 their growth, thus giving to the umbilicus a decidedly embryonic 

 aspect. Envelopment extends laterally to the geniculre. 



Lower Lias. 



A. KRIDION, d'Orb. T. 106, figs. 605, 606. 



OPHIOCERAS. Keel constant, sometimes obscure. The shell 

 has a greater number of whorls than in the preceding genus, 

 because the young increase more slowly in size. Pilre straight, 

 depressed, appear at an early stage in the young, and are well 



