718 



Report of the State Geologist. 



4. It becomes further evident from the study of the minute shells that 

 the latter were round and that the marginal grooves were only part of a 

 superficial layer. The fact that such multitudes of the youngest shells could 

 be so well preserved while the more advanced stages become rarer in propor- 

 tion to their size, as well as the excellent state of preservation of the former, 

 can not but suggest that the shells were relatively much stronger in the 

 younger stages than in the later ones. On the other hand, a comparison 

 between the young and old forms impresses upon the observer the greater 

 relative stoutness of the marginal grooves in the latter. They are for the 

 most part hardly noticeable in the infantile stage, and when the specimens 

 are flattened they have often no effect upon the outline which is then curved 

 (cf. PI. IV, figs. 5, 10, 14, 17, 18, 21, 28, 32). It can, therefore, be said that 

 while the younger stages of growth had stouter walls and less prominent mar- 

 ginal grooves, with advancing age the walls of the faces became relatively thin- 

 ner and the Avails of the marginal grooves stronger. This gives the impression 

 that there had been a tendency to make the shell lighter and at the same time 

 to preserve its strength ; a tendency which would be only of advantage in 

 changing a sessile mode of life either to a creeping or swimming one. Thus 

 it seems that the well-known fact of the thinness of the shells of most species 

 of Conularia w hich suggest a swimming mode of life, and the observation of 

 the attachment of the rather stout shells of our species in its infantile and 

 adolescent stages, can be made to harmonize by assuming that Conularia 

 represents a group of animals which were in the process of adapting them- 

 selves from a sessile to an errant mode of life. It is probable that in most 

 forms, especially those of the Devonian and Carboniferous ages, the onto- 

 genetic course of development was materially shortened ; hence the absence 

 of such developmental stages as Conularia gracilis has furnished. 



It has already been shown by the structure of the basal appendage 

 and the mode of occurrence of the growth stages possessing it, that these 

 youthful forms were also capable of some change in place of attachment. 



The slight development of the surface sculpture in the young shells and 

 the preponderance of the transverse ribs over the longitudinal ones in some 

 of the best specimens (cf. PI. IV, figs. 32 and 35) are perhaps also characters 

 of some phylogenetic significance. 



Granted, then, that the described features of the young of Conularia 

 gracilis are of a palingenetic character, it would follow that this Conularia 

 took its origin from animals with slender, conical, little sculptured, 

 chitinous shells, which were attached by basal, cup-like appendages. 



