224 TOWARDS THE HUMAN FORM 



to be continuous but straight. This decides the place that 

 Hyolites and Tentaculites should occupy, which, if this 

 deduction is not followed, are sometimes classified among the 

 Annelid Worms. The phase of spiral formation is represented by 

 Bellerophon. All these forms are Cambrian, and with them we 

 must include the helicoidal forms of the Diotocardi ac Gastropods, 

 whose primitive characters we have already described, as well 

 as Euomphalns, and the Pleurotomarice. These last persist 

 to the present day in the deep-sea fauna. From the Silurian 

 onwards may be added such forms as Trochus, also diotocardiac, 

 Patella, transitional forms representing the first Monocardiacs 

 with shells entirely open, 1 the carnivorous Purple-Fish, 

 whose shell is channelled at the opening in order to permit 

 the passage of a siphon destined to bring the water into the 

 branchial cavity. The Turbos, differing from Trochus in 

 the thickness of its calcareous operculum, and Capulus with 

 its small, hood-shaped, scarcely pointed shell, appear in the 

 Devonian. The Carboniferous witnesses the arrival of the 

 " Worm-shell ", which attaches its shell to foreign bodies, and 

 the first true Snails and other pulmonata Gasteropod, such 

 as the Pupae. Seeing that the lake deposits and river drift 

 of the Carboniferous are so incomparably better known than 

 those of the preceding periods, it is conceivable that the 

 pulmonate type had been achieved much earlier. 



The straight-shelled Cephalopods are represented by the 

 Orthoceratidae. The Cephalopods retained their swimming 

 habits, and when their shell became rolled it simply formed 

 a spiral and remained symmetrical. Both types, at the latest, 

 existed in the Silurian. It was not until the Cretaceous that 

 certain Ammonites, which probably became crawling organisms 

 for reasons we have already given (p. 137), acquired the 

 helicoidal form, and thus constituted the Turrilite family. 

 The straight-shelled and related forms enable us, moreover, 

 to divine the causes that determined the special characters 

 of Cephalopod shells. These shells are divided internally into 

 successive chambers by calcareous septa, concave near the 

 shell orifice, and either attached to its walls in a gradual 

 manner so as to preserve their curvature — a characteristic of 

 the most ancient forms, constituting the group of Nautilidas 



1 Littorinidae, Scalarid?e, Pyramidellidse. 



