CEPHALOPODA FROM THE HOKKAIDO. 23 



We do not know any morphological equivalents of this 

 peculiar shell among similar tube-forming animals of the past 

 ■or the present. The following cases of abnormal growth of the 

 shell of lurrilites and its allied genera may be taken into con- 

 sideration. 



(1.) The Ammonites with a conically spiral shell usually 

 follow a certain law in their growth, having volutions which are 

 •either mutually in contact or separate ; but cases often occur in 

 which they show a portion near the mouth more or less per- 

 ceptibly deflected in position from the preceding volutions. This 

 -change may be produced by the animal either pathologically or 

 as the result of old age. AVe also learn from various sources, 

 the occurrence of many examples in discoidal species, showing 

 a slight tendency to become conically spiral in the anterior por- 

 tion. QuENSTEDT and some others early called attention to this 

 fact and seem to have believed the probable derivation of second- 

 ary forms from ancestral individuals of pathologic growth. 



(2.) In some of the secondary forms, it is known that at 

 the very beginning of development they show a normal growth, 

 that is to say, a discoidal shell with volutions more or 

 less involute. A remarkable case has recently been described 

 by Whitefield^^ in Heteroceras sinipUcostatum from the Fort 

 Benton group of the Black Hills. According to this author, 

 the species has its younger portion composed of two straight 

 limbs, close together, like Hamites. This seems to indicate the 

 derivation of a more abnormal type from a less abnormal one. 



(3.) Besides, there are a few cases in which the shell is 

 coiled in a conical spiral, first to the right and then to the left. 



1) ^VHITEFIELD : Observations on and emended Descriptions of Heteroceras simpHcostahim 

 "WiiiTKFiELD. (By review.) 



