550 MOLLUSCA SUB-KINGDOM VI 
entirely oceupied by a large azygous lobe terminating in a minute annular lobe and 
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partial cone similar to that observed in Nautiloids and Pinnacites. The imner 
extension or cone is not present elsewhere among Ammonoids so far as known. 
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Devonian. 
Family 4. Pinnacitidae. Highly involute, compressed ammoniticones with 
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acute venters. Septa biconcave, owing to division of the lateral lobes by narrow 
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saddles which are connected by ridges with corresponding saddles on the dorsum. 
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There are also saddles at the umbilical angles and on either side of the ventral lobes. 
. . to) . . 
The azygous dorsal lobe is large and similar to that of Aphyllitidae. Dorsum with one 
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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 C. MESOCAMPYLI. Hyatt. 
A provisional group including a few forms which exhibit a wide range of sutwral 
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 ina 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 Neotcoceras, gen. noy., which 
combines the lateral sutures of Anarcestes with the 
divided ventral lobes of the Primordialidae, ut the 
siphonal saddles are large and entire. Volutions 
diseoidal and trapezoidal in section, similar to the 
young of Gephyroceras caleuliforme, Sandb. sp. 
Position of siphuncle unknown. Type P. (Gon.) 
elkhornensis, Mill. and G. sp. Carboniferous ; 
I1hnois. 
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 
Fic. 1125. formed by division of the first lateral saddles. Septa 
Gephyroceras intumescens, Beyr. Upper in the young concave and similar to those of Anar- 
Mae heer eh se GN 2 Ua al cestes, but in later stages become convex along the 
mesal plane as in normal Ammonoids. Siphuncle 
small, subventran, without calcareous sheath ; funnels monochoanitie and short. 
Gephyroceras (Fig. 1125), Manticoceras, Hyatt; Probeloceras, Clarke ; Timanites, 
Mojs. Devonian. Milleroceras, gen. nov. Type M. (Gon.) Parrishi, Mill. and G. sp. 
Upper Coal Measures ; Ilinois. 

