CLASS V 



CEPHALOPODA 



623 



are repeated in regulär succession during the ontogeny of chloiochoanitic forms 

 (Fig. 1149) except when accelerated development (tachygenesis) occurs, and 



Fio. 1149. 



Tropites cf. phoebus Dittm. Trias. Bn- 

 larged section in the median plane of the 

 young, showing retro.siphonate funnels in 

 the nepionic stage, then transitional, and 

 later prosiphonate funnels. a, Position of 

 protoconch (after Branco). 



Amaltheus spinatus Brug. Lias. 

 Section parallel to median plane, show- 

 ing Position of the siphuncle. a, Pro- 

 toconch ; c, Caeciim (after Branco). 



then the monochoanitic stage may disappear. The reduction in size of the 

 siphuncle among Ammonoids is obviously correlated with loss of functional 

 importance, as is also the case among more specialised Nautiloids ; and con- 

 sequently organic deposits are not found in the caraerae of these shells. 



Living Chamber. — This varies greatly in 

 all of its dimensions, thus indicating difFer- 

 ences in the size and proportions of the 

 animal, since its body parts were probably 

 wholly contained within this cavity. The 

 lines of growth and the few apertures known 

 among Nautilinidae and Clymeniidae show 

 that they had hyponomic sinuses on the 

 venter, and were swimmers like Nautiloids. 

 The same was probably true of the Gephyro- 

 ceratidae, except during the gerontic stage of 

 some species when a ventral crest arises, as Fig. 1151. 



demonstrated by Clarke. In the Glyphio- ParUnsonia parkinsoni (Sowerhy). Middle 



ceratidae and Beloceratidae many species that wShuib^lTenirrglTenUcrprlsip^^^ 

 retain the so-called goniatitic form have «ndposition of protoconch (a) ^^^^^ 

 hyponomic sinuses, but occasionally short 



ventral crests appear, and later these become general. Only radical 

 Paleozoic forms of the-Arcestidae have retained the hyponomic sinus ; short 

 obtuse crests appear in the Trias, and continue thereafter. Jurassic and 

 Cretaceous Ammonoids have as a rule more pointed rostra than those of the 

 Trias, and frequently develop lateral crests and lappets (Figs. 1152, 1156). 



Very decided decrease in the dimensions of the living chamber during the 

 senile stage does not occur as a rule among Paleozoic forms ; but this con- 

 dition appears among the Triassic Haloi'itinae and Tropitidae with a corre- 

 sponding contraction of the aperture. The Arcestidae (Fig. 1222) and some 

 species of the Ceratitidae also often have very narrow openings during the 



