i50 



MOLLUSCA 



SUB-KINGDOM VI 



entirely occupied by a large azygous lobe terminating in a minute annular lobe and 

 partial cone similar to that observed in Nautiloids and Pinnacites. The inner 

 extension or cone is not present elsewhere among Ammonoids so far as known. 

 Devonian. 



Family 4. Pinnacitidae. Highly involute, compressed amrnoniticones with 

 acute venters. Septa biconcave, owing to division of the lateral lobes by narrow 

 saddles which are connected by ridges with corresponding saddles on the dorsum. 

 There are also saddles at the umbilical angles and on either side of the ventral lobes. 

 The azygous dorsal lobe is large and similar to that of Aphyllitidae. Dorsum with one 

 pair of narrow zygous saddles and one pair of broad zygous lobes, giving a formula of 

 eight lobes and eight saddles. 



Includes only Pinnacites, Mojs., from the European Devonian. 



Sub-Order 0. MESOCAMPYLI. Hyatt. 



A provisional group including a few forms which exhibit a wide range of sutural 

 modifications, as well as certain genera intermediate between the nautiliform Anarcestes 

 with concave septa, and the normal Ammonoidea with convex septa and large lateral 

 saddles. Ventral lobes invariably divided; siphonal saddles entire in some primitive 

 genera and in a few sporadic forms among the more specialised species. Aperture with 

 hyponomic sinus during the later stages of ontogeny, but in some forms a ventral crest 

 appears during the paragerontic stage (Clarke). 



The external sutures sometimes approximate to those of Anarcestes, but the dorsals 

 have only one large azygous lobe, the saddle being confluent at the line of involution 



with the second external pair. In more involute 

 forms the antisiphonal lobe is large, entire, and 

 pointed ; there is one pair of dorsal saddles, and one 

 of broad dorsal lobes; the second pair of saddles, 

 when present, is confluent with the second pair of 

 lateral saddles. 



Family 1. Neoicoceratidae. This in- 

 cludes the unique Neoicoceras, gen. nov., which 

 combines the lateral sutures of Anarcestes with the 

 divided ventral lobes of the Primordialidae, but the 

 siphonal saddles are large and entire. Volutions 

 discoidal and trapezoidal in section, similar to the 

 young of Gephyroceras calculiforme, Sandb. sp. 

 Position of siphuncle unknown. Type P. (Gon.) 

 elkhornensis, Mill, and G. sp. Carboniferous ; 

 Illinois. 



Family 2. Primordialidae (Primordiales, 

 Beyr.). Distinguished from Anarcestes by the divided 

 ventral lobes, large siphonal saddles, and especially 

 the first lateral saddles, which are very prominent 

 on the sides. Adventitious lobes and saddles are 



FIG. 1125. formed by division of the first lateral saddles. Septa 



intumescens, Beyr. Upper in the young concave and similar to those of Anar- 

 ~sau. A, Conch, Vi- ^, cgg ^ k ut in later stageg become convex along the 



mesal plane as in normal Ammonoids. Siphuncle 

 small, subventran, without calcareous sheatli ; funnels monochoanitic and short. 



Gephyroceras (Fig. 1125), Manticoceras, Hyatt; Probeloceras, Clarke; Timanites, 

 Mojs. Devonian. Milleroceras, gen. nov. Type M. (Gon.) Parrishi, Mill, and < :. >i>. 

 Upper Coal Measures ; Illinois. 



