I'KIIONOCKKAiS, TIIYSA N(»('KRAS, ETC. '22*.) 



poinU'd, ajul in a single row. Sei)tal lobes with numerous 

 pointed, deeply cut, irregularly shaped minor lobes. Abdominal 

 lobe very deep, and level with superior lateral lol)e. Siphonal 



cell long and narrow. 



Loicer, Middle and Upper Lias. 



D. ziPHius, Ziet. T. 107, fig. (311. 



Peronoceras. Abdomen depressed ; pilse depressed ; linear 

 between the tubercles ; usually, but not invariably bifurcated on 

 the abdomen. Tubercles depressed, often obtuse upon the casts, 

 but pointed and prominent upon the shell. Septa not closely 

 crowded, as in Deroceras, or so profuseh' branching. 



Middle Lias. 



P. MUTicus, d'Ovb. T. 1U8, figs. 622, 023. 



Ftimily THYSANOID^.. This family includes the Fimbriati, 

 Ligati, and Heterophylli, which agree in the foliaceous char- 

 acter of the septa. 



Thysanoceras.* Abdomen rounded ; whorls exposed ; the 



envelopment does not extend laterall}' over more than o^e-third 



of each interior whorl. Abdominal lobe about the same depth, 



but narrower than the superior lateral lobe ; the latter is equally 



divided by a peculiar minor cell of a lobiform aspect. The 



siphonal cell is cuneiform, and the superior and inferior lateral 



cells equally divided. 



Middle and Upper Lias. 



T. FiMBRiATUS, Sowb. T. lOl, figs. 538, 639. 



RHACOCERAS.f Abdomen rounded ; sides of the whorls flat- 

 tened ; envelopment extends over about two-thirds of each of 

 the interior whorls, or entirely encloses them, covering up the 

 umbilicus. The lobes and cells gradually decrease in size in- 

 wardl}', and are remarkable for the profusion and peculiar folia- 

 ceous aspect of the minor cells (= section Heterophylli). 



Middle and Upper Lias. 



R. HETEROPHYLLUS, Sowb. T. 102, figs. 544, 545. 



Professor Meek includes a number of American eretaceous 

 species. 



"* Syn, of Lytoceras, Suess. Hyatt, Bull. Mus. Comp. Zool., iii, 59. 

 f Syn. of Phylloceras, Suess. Ihid. 



