Clarke — The Naples Fauna. 



115 



3 "4" wm 

 1 3 *iri rrv. 



3 T-n TTV. 



In its history the suture passes through simple changes. Beecher has 

 shown that the first suture is an almost direct line without evidence of ventral 

 lobe, and that on the second septum this lobe is indi- 

 cated and accompanied by a slight forward lateral 

 curvature forming primitive saddles. When the 

 shell has a diameter of 3 mm. w e find that the suture 

 shows a division of this primitive saddle by a broad 

 ventrolateral lobe, thus giving rise also to the lateral 

 saddle. At 3.5 mm. diameter, the prominence of the 

 lateral lobe and this tendency constantly progressed 

 render the lobe more conspicuous and deepen the 

 large saddles. Progress in the form of the suture 

 hereafter consists in a more pronounced development 

 of these curves, the narrowing of the lateral saddles 

 and increase in convexity of the outer slope of the 

 lobes. 



Figure 94. Tornoceras uniangulare. 



Ornamentation. The fine transverse striations nw^****^"^ 

 of the protoconch which have been observed by Beecher are not present 

 in our specimens. The ornamentation of later growth stages, though almost 

 invariably obscure, consists of fine concentric lines which are elevated, sharp 

 and simple over the earlier whorls but soon become fasciculated and obso- 

 lescent so that in the mature condition the whorls are virtually smooth 

 on the exterior, though sharp internal casts may retain traces of the curving 

 striae and also show obscure lateral revolving bands which are not represented 

 on the exterior. The external striae make a broad anterior sweep from the 

 umbilicus and with a rather abrupt and acute turn recurve into a wide and 

 deep hyponomic sinus. The narrow anterior curves which bound this sinus lie 

 in two revolving furrows which are plainly visible on the third and fourth 

 whorls, where they outline a ventral ridge, but ridge and furrows become 

 rapidly obscured on later whorls. Very young shells in the second and third 

 volutions sometimes bear transverse labial ridges, which are never seen in 

 adults of American species but appear commonly among European specimens 

 of this genus. The character of the inner shell layer, or " Runzelschicht," in 

 this genus has been illustrated by both Hall* and Holzapfel.+ In the 

 umbilical region where the shell has become calloused this layer bears fine, 

 implanted or anastomosing raised lines concentric to the umbilicus. These 



* Palaeontology of New York, vol. v. pt. 2, Suppl. (=vol. vii), pi. cxrvii, fig. 12, 1888. 

 t Das obere Mittel-devon im Rhein. Gebirge, 1895. 



