226 TOWARDS THE HUMAN FORM 



It must have been a sort of a tail whose formation would be 

 mechanically conditioned by the weight of the shell hanging 

 in the water. Once formed, this siphuncle would be hereditarily 

 preserved in the later spiral forms. This implies that the part 

 of the Mollusc's body containing the viscera mounts up within 

 the shell step by step as it grows, and secretes behind it, 

 at each stage, a partition isolating it from the empty part of 

 the shell. The siphuncle remains pressed against the outer wall 

 of the shell in the Goniatites ; it is internal in the Clymenise, 

 a group which lasted but a short time. The same theory that 

 accounts for the origin of the Cephalopod Molluscs finds a 

 natural extension here. We shall probably never know 

 exactly how Orthoceras was formed, but it is impossible to 

 doubt its genealogical relationship with the other shelled 

 Cephalopods. 



From the Silurian Period the septa begin to become 

 sinuous in Goniatites of the genera Anarcestes and A goniatites. 

 These continue to grow more complicated, each series 

 being characterized by the relative proportions of height, 

 width, and length of the chambers, particularly of the last, 

 which is necessarily moulded upon the body of the mollusc. 

 Von Mosjisowicz, En. Kayser, Fr. Frich, Emile Haug, etc., 

 have been able to follow the gradual evolution of the diverse 

 series of Ammonites, thus contributing important evidence to 

 demonstrate the theory of evolution. 



From the general point of view, at this stage, there is little 

 to be said about the Lamellibranch Molluscs. They began 

 in the Lower Cambrian, which indicates that the symmetrical 

 Diotocardiac Gasteropods, which evolved into Bellerophon, must 

 have already existed during the Pre-Cambrian Period. Not 

 until the Silurian, however, do their species become sufficiently 

 numerous to enable us to follow the successive stages of their 

 evolution. As theory indicates, the oldest forms have a long 

 hinge with a very simple articulation ; these form the group 

 of " Pakeochonchae ". 1 Then come the genera in which the 

 very long hinge has numerous small close-set teeth, Cucullella, 

 Leda, etc. ; and, following them, species which suspend 

 themselves by a byssus, and whose shell becomes broadened 

 near the base on account of its own weight, of which the muscles 



1 Cardiola, Conocardium, Dualina, Lunulocardium, Prcecardium, Slava, 

 Vlasta, etc. 



