INVERTEBRATE PALAEONTOLOGY. 491 



3. trbmatodiscus, Meek and Worthen. 



Shell discoid, with umbilicus very wide, usually shallow, and 

 showing all of the inner turns, generally perforated; volutions narrow, 

 increasing gradually in size, very little or not at all embracing, and 

 provided with several revolving angles and furrows, usually both on 

 the sides and periphery, sometimes also marked with revolving striae ; 

 septa merely a little arched. — (Nautilus stygialis, de Koninck.) 



■A. discites (De Hann), McCoy. 



Shell discoid, with umbilicus usually wide, shallow, and show- 

 ing all the inner turns ; volutions narrow, increasing gradually in size, 

 tpiadrangular, sometimes nodifcrous, not embracing ; septa straight 

 or arcuate on their lateral margins and the flattened or, concave 

 periphery ; surface showing marks of growth. — (Nautilus planitur- 

 gatus, McCoy.) 



5. solenochilus, Meek and Worthen (= Crijptoceras, d'Orbigny). 



Shell generally presenting the form and surface-characters of 

 Nautilus proper, but with siphuncle very nearly against the periph- 

 ery, and the lip on each side near the umbilicus drawn out into a 

 narrow, spout-like projection. — (Nautilus (Crijptoceras) sjnniger, 

 White and St, John.) 



6. hercoglossa, Conrad (— Agrmides, Montfort "?). 



Shell more or less discoid, with umbilicus closed or small, and 

 periphery usually rather narrowly rounded ; volutions deeply embrac- 

 ing, surface nearly smooth, or with lines of growth; septa deflected 

 backward in crossing each side, so as to form a deep, usually angu- 

 lar lateral lobe* — (Nautilus orbiculatus, Tuomey ; N. Parkinsoni, 

 Edwards, &c.) 



7. pseudonautilus, Meek.f 



Differs from the last in having the septa also provided with a 

 well-defined peripheral and antiperipheral lobe, and the siphuncle 

 placed near the outer margin. — (Nautilus Geinitzi, Oppel.) 



* In some Triassic species that seem to fall into this group, such as N. noricas, Mojsisovics, there 

 are, as already intimated, two of these lobes on each Bide. 



tThis type presents very curious and interesting intermediate characters between the Nautilus 

 and Goniatites groups ; that is, it has not only the lateral lobes of the septa of the latter, as we see in 

 some other NautiUnd groups, but also tin- peripheral, or so-called dorsal lobe of Goniatites, while its 



