II VKI 



Si phonal lobe ;i I most afl deep M wide ; t he point o|' :it t a eh merit 



be siphon is e\aetl\ in the middle of [ti depth ; tin- upper 



lateral does in ,1 :itl:iin li;ilt' of its depth and is :it le;i-t M bro.-nl 



leep; tin- lateral saddle is more elevated Mum :dl theothei- 



:ind -t.-iiHK above the ba>e of tin- upper lateral usually double M 

 high a- the external saddle; the h>\\er lateral lobe is much 

 broader than deep ftnd (he antisiphoiial saddle so -mall, that it 

 doe- not attain half 1 he height and width of the lateral saddle. 

 Ant i>iphonal lobe t wo-pointed. Corneou-. simple anapt \chn-. 



Waagen -a\> that the separat ion of . I //////> and .K<inr,-r<t.< 

 18 difficult, and Hyatt oliscrxcs that a ^eiietie interrelationship 



exists between the two. The first representative^ appear in the 



lowermost Lias, and aceordhii>- to our present understanding c,f 

 the ^-niis they appear to have died out in the lower Lias, though 

 many forms appear, which at present referred to Ilarpoceras, in 

 reality belong to Arietites. as, for exaiujde, 7/<ir/>. Algoviamum. 



Thirty-eiii'lit species. 

 ARIKTITES OPHIDIOIDES, Orb. T. 107. figs. 607, 608. 



SCIPIONIANUS, Orb. T. 108, figs. 629, 630. 

 " OBTUSUS, Sowb. T. 107, figs. 614, 615. 



KiticiON, Orb. T. 106, figs. 605, 606. 



BISULCATUS, Brong. T. 103, fig. 557. 



Genus HA.RPOCERAS, Waagen. 



External form of the shell variable, external side always eari- 

 nate or aiiunlar; >eiil})ture consisting of more or less distinct 

 sickle-like ribs. Margin of aperture sickle-shaped, or with ears, 

 with pointed external lobes; body-chamber embracing one-half 



to two-thirds of a whorl, earinate to the mariiin of the aperture. 

 Aptyclms divided, thin, calcareous, with a thick, shelly layer. 

 more or less folded. 



Lobes mostly not deeply notched, always two lateral lobes and 

 almost always auxiliaries. Siphonal lobes ending in twodi\ 

 inu' hranche-. usually >horter than the first lateral: laterals not 

 divided into -y inmet rical hal\ 



Sharp demarkat ion from t he LL'i'iiu- ./ is \\anl in^. sinet- 



the forms from the group iuelmling .K<j. <i ri>'l ij'rni''. Opp.. tit a- 

 well into the one a- into the other ; thenio-t recent //'/, 

 is llnrii./.in from the upper K imnieriduiian. The piv-eiit 



