480 FREDERICK W. SARDESON 



same cause could not operate well because of change of poise or 

 relation of shell to the body, and the acuminate aperture may 

 not remain evident where the curvature has changed. The 

 curvature may be taken as backward or lateral, and backward 

 not forward, by rule, since the exception in Hypseloconus is 

 seen to be a reversal {vide H. recurva Whitf., Plate I, Fig. 5). 

 It is also notable that the Hyolithes curve backward when at all, 

 and a reason for considering the angular side of Hyolithes as 

 the posterior one may be deduced also from the triangular 

 cephalopods. The initial of Endoceras is hyolithoid-shaped and 

 angular dorsally; and, indeed, the frequent angular feature in 

 early Cephalopoda, such as Gonioceras, and Triptoceras suggests 

 that the oval acuminate aperture may have been ancestral. The 

 cephalopod shell may have derived the marine hyolithoid spe- 

 cies by formation of lateral loculi, in the manner described by 

 Holm, in place of terminal ones, and the descent may have been 

 in Cambrian time from Gastropoda to Hyolithoidea and to Cepha- 

 lopoda, whence an ancestral trait of the oval aperture. Further, 

 Cephalopoda and Hyolithoidea have not a coiled embryonic 

 shell. Gastropods in the Cambrian should not, then, have, and 

 are not known to have. 



The curvature or coiling in the apex of the early Cambrian 

 Gastropoda appears either equal to or greater than that in the 

 maturer shell {^Platyceras primaevum, Fig. 14), and correspond- 

 ingly we may outline the morphologic series from that of the 

 long conical to the apically curved, the apically coiled with 

 curved base, and the entirely coiled, in accordance with the 

 theory advanced, i. e., that the shells of Cambrian time were 

 coiling and not uncoiling. That these belong to an uncoiling 

 stage would be tenable only on the evidence of an unknown 

 fauna of spiral shells as old as, or older than, this one. Of that 

 there is no proof. We do not know that there was a Cambrian 

 larval spiral protoconch. In Recent shells such may have 

 resulted altogether as it has in part since then. And, as said, 

 the twisted mantle complex may have required no further cause 

 than the shifting of a long cone. Taking the theory that the 

 long conical shell, in Cambrian time, was ancestral to both the 



