2'2(\ PSILOCERAS, ARNKM'KKAS. OPHIOCERAS. 



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 " genicula? " 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 Pr<><-.. 

 955, 18T8. 



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 geniculie. 



Lower Lias. 



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



OPHIOCERAS. Keel constant, sometimes obscure. The shell 

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

 because the young increase more slowly in size. Pila 1 straight, 

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



