METAZOA MOLLUSCA. 215 



sulcomarginata) occurs, which was a coiled shell consist- 

 ing of a few whorls with a slit in the margin of the 

 aperture, that was either filled as the animal grew older, 

 making a continuous band, or partly filled giving rise to 

 openings. 



In the Silurian and Devonian ages, Platyceras (No. 437, 

 P. erectum Hall) existed, the apex of which was twisted 

 so as to form a spiral while the later whorl was flaring, 

 showing a tendency to uncoil. an old age or gerontic 

 character. 



In the group of Gastropods represented by Tryblidium, 

 Pleurotomaria, and their descendants Patella, Haliotis, 

 etc., there seems not to be any primitive genus with a 

 cap-like shell in the adult stage. Neither is there a genus 

 with the loosely-coiled shell, the transitional form between 

 the cap and the close spiral, such as is found in the class 

 of Cephalopoda. 



Both Patella and Fissurella were formerly supposed to 

 be primitive members of the group to which they belong; 

 but in reality they are found to be specialized forms. 

 This is proved by their development, which in the case 

 of Fissurella has been figured and described from the egg 

 to the adult stage. 1 



Since the adult Patella is less modified than the adult 

 Fissurella, it will be briefly described. 



We pass over the development of the egg and the for- 

 mation of the embryonic nautiloid shell figured by Patten, 2 

 and come to the patelliform stages of the shell. In one 

 of the simplest Patellidae, Acmaea, the shell is without 

 ribs, spines, or ornaments of any kind, and one species, 

 A. punctulata Gmel., is conical when young though it is 

 depressed like most of its group when full grown. 



In Helcioniscus exaratus Nutt., often called Patella 

 (No. 438), the perfect shell is smooth at the apex, but 



1 Boutan, Arch, de Zool. Exper. et Gdn., ser. 2, III, suppl., 1885. 



2 Arbeit, zool. Inst. Univ. Wien, VI, 1886. 



