CLASS V 



CEPHALOPODA 



619 



Fig. 1140. 



Angustisellate stage of Phyllocercwi 

 heteropJiyllum (Sowb.). Lias. 



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



desci'ibed in a masterly way by Branco, are divided by him into thiee classes : 



asellate, latisellate and angusfisellate (Figs. 



1138-11 40). The asellate cross the venter as 



a straight line or very slight saddle, and are 



present only in the ephebic stages of Cyrto- 



chjmeniaCl) 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 corresponding 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 Branco, 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 

 ^ ji c throughout. The 



embryosof 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 herein after feferred to as the ventral lohe, and by a 



^ The terminology comnionly in vogue designates the sutural inflectioiis as follows : — The ventral 

 or external lobe is bounded on eitlier side of the mesal plane by the \a.vge ßrst or mpeincn'-lateral 

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

 lateral saddle and lobe in tlie order nanied. All additional inflections occurring ])etween the second 

 lateral lobe and the line of Involution are termed auxüiaries, and are nntnbered in replar order. 

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

 nnderstood the angulated or digitated portions of the snture which are directed backicards, away 

 froni the niouth of the shell ; " saddles " are the elevations between them, which point tmcards the 

 aperture of the shell. 



^^WxATN 





A, Development of sntures in a latisellate Qoniatite, Gastrioceroft dioxlrma 

 (Goldf.). Lower Carboniferous ; Choquier, Belgiiini. B, Same in a latisellate 

 Ammonite {Tropites suhhuUatiis Hauer). G, Same in an angustisellate Ammonite 

 (all after Branco). Sutures of the first volution are lettered consecutively from 

 <i toi; those of the second from m to s. 



