538 



MOLLUSCA 



SUB-KINGDOM VI 



FIG. 1091. 



Angustisellate stage of Phylloceras hetero 

 phyllum, Sow. sp. Lias. 



septum closed the aperture of the protoconch, and the caecum projected into 

 its interior. The caecum is connected with the internal surface of the proto- 

 conch by bands (Figs. 1101, 1102), or semiconical prolongations, described 



by Munier-Chalmas as the prosiphon. But 

 these bands are of various shapes, are not 

 connected with the interior of the caecum, 

 and appear to be merely calcareous supports 

 for the bottom of the caecum. The earliest 

 sutures, described in a masterly way by 

 Branco, are divided by him into three classes : 

 asellate, latisellate, and angustisellate (Figs. 

 1089-1091). The first cross the venter as a 

 straight line or very slight saddle, and are 

 present only in the ephebic stages of Cyrtodymenia (?) and in the mimoceran 

 stage of the Microcampyli. In all except primitive forms it is confined (as are 

 most of the purely nautiloidean characters) to the first septum. The latisellate 

 stage is characterised by a decided broad saddle on the venter, with corre- 

 sponding 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 Gastrocampyli, according to Branco, but his figure shows a 

 saddle on the venter. The Microcampyli and Mesocampyli are asellate, and the 

 Eurycampyli also in A R c 



some primitive De- 

 vonian genera, but 

 latisellate in others, 

 and angustisellate 

 in the Trias. The 

 embryos of primi- 

 tive Phyllocampyli 

 are unknown, but 

 the Triassic Lobi- 

 tidae and Arcestidae 

 are latisellate, while 

 the Cladiscitidae and 



,i r>j 77 , . 7 Goldf. sp.). Carboniferous Limestone ; Choquier, Belgium. B, Same in a latisel- 



tne JrliyliOCeraiiaae late Ammonite (Tropites siibbullatits, Hauer). C, Same in an angustisellate 



nrp ano-n<5ripllatp Ammonite (all after Branco). Sutures of the first volution are lettered con- 



6 secutively from g to I ; those of the second from m to s. 



throughout. The 



embryos of Discocampyli are almost unknown, but are supposed to be latisellate, 

 with the exception of the highly specialised Pinacoceratidae, which are angusti- 

 sellate. The remaining sub-orders are wholly Jurassic and Cretaceous, and so 

 far as known, the first septa are angustisellate. 



Sutures. 1 The second septum (Fig. 1092) in all but the most primitive 



1 [The nomenclature commonly in vogue designates the sutural inflections as follows : The ventral 

 or external lobe is bounded on either side of the mesal plane by the large first or superior -lateral 

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

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



FIG. 1092. 

 Development of sutures in a latisellate Goniatite (Glyphioceras diadema, 



