CLASS V 



CEPHALOPODA 



619 



Pig. 1140. 



Angustisellate stage of PhyUocnras 

 hetewphyllum (Sowb.). Lias. 



calcareous supports for the bottom of the caecum. The earliest sutures, 

 described in a masterly way by Braneo, are divided by him into three classes : 

 asellate, laiisellafe and angustisellate (Figs. 

 1138-1140). The asellate cross the venter as 

 a straight line or very slight saddle, and are 

 present only in the ephebic stages of Cyrto- 

 dymenia{1) and in the young mimoceran stage 

 of the Nautilinidae. In all except primitive 

 forms it is confined (as are most of the 

 purely nautiloidean characters) to the first 

 septum. The latisellate stage is character- 

 ised by a decided broad saddle on the 

 venter, with corresponding deeper and broader lobes on the sides. The 

 angustisellate stage has prominent, sometimes almost sub -acute ventral 

 saddles with corresjDonding deep lateral lobes, accompanied by definite saddles 

 at the umbilical depressions. 



The last two stages are progressive modifications confined to the larvae of 

 Ammonoids, and are not present in the ephebic stages of any known species. 

 The asellate condition of the first septum is found in the ananepionic stage of 

 one species of the Clymeniidae, according to Braneo, but his figure shows a 

 saddle on the venter. The Nautilinidae and Gephyroceratidae are asellate, and 

 the Glyphioceratidae also in some primitive Devonian genera, but latisellate in 

 others, and angustisellate in the Trias. The Triassic Lobitidae and Arcestidae 

 are latisellate, while the Cladiscitidae and the Phylloceratidae are angustisellate 

 4 P, c throughout. The 



embryos of theCera- 

 tidae are very little 

 known, but are sup- 

 posed to be latisel- 

 late, while the 

 highly specialised 

 Pinacoceratidae are 

 angustisellate. The 

 remaining system- 

 atic groups are 

 wholly Jurassic and 

 Cretaceous, and so 

 far as known, the 

 first septa are an- 

 gustisellate. 



Sutures} — The second septum (Fig. 1141) in all but the most primitive 

 forms becomes divided by an entire azygous lobe on the venter, often termed 

 the "siphonal lobe," but hereinafter referred to as the ventral lobe, and by a 



^ The terminology commonly in vogue designates the siitural inflections as follows : — The ventral 

 or external lobe is boinuled on either side of the niesal plane by the laxge. first or superior -lateral 

 saddle. This is followed liy the first or snperior-lateral lobe, and then come the second or inferior- 

 lateral saddle and lobe in the order named. All additional inflections occurring between the second 

 lateral lobe and the line of involution are termed auxiliaries, and are numbered in regular order. 

 The antisiphonal is also known as the internal, dorsal or columellar lobe. By "lobes" are always 

 understood the angulated or digitated portions of the suture which are directed backwards, away 

 from the mouth of the shell ; "'saddles" are the elevations between them, which point towards the 

 aperture of the shell. 



'■^VWV^: 



HrvAAAf " 



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Gastrioceras diadcma 

 Same in a latjsellate 



Fig. 1141. 



A, Development of sutures in a latisellate Qoniatite, 

 (Goldf.). Lower Carboniferous ; Choquier, Belgium. B, 



Ammonite {Tropitcs fuhbuUatiis Hauer). C, Same in an angustisellate Ammonite 

 (all after Braneo). Sutuies of the first volution are lettered consecutively from 

 y to I; those of the second from m to s. 



