8o6 



DIVISIONS OF THE GASTROPODA. 



Fig. 720. — Thin section of limestone 

 from the Devonian rocks of Canandaigua, 

 North America, composed almost entirely 

 of the tubes of Styliola Jissurella, en- 

 larged twenty times. (Original.) 



denies that the so-called " Styliolae " of the Silurian and Devonian 

 rocks can be referred to the Pteropoda on the grounds of their great 



size, the absence of an embryonic 

 shell, and the fact that no existing 

 species of Clio has the shell longi- 

 tudinally striated. The first two of 

 these objections, however, do not 

 apply to Styliola fissurella and its 

 allies, since the tube is smaller than 

 that of some of the existing species 

 of Clio, and an embryonic shell is 

 present ; while the third objection 

 would appear to be of anything but 

 a conclusive character. It is cer- 

 tainly remarkable that, admitting the 

 Pteropodal nature of Styliola fissur- 

 ella, no traces of forms of the same 

 genus should have hitherto been met 

 with in the later Palaeozoic and in 

 the Secondary deposits, and that the 

 genus is not known to be again 

 represented till the Miocene period 

 is reached. Palaeontology, however, 

 is rich in examples of gaps of this kind, and little importance can 

 be attached to a seeming hiatus, which may any day be partially 

 filled up. 



Of the remaining genera of the Cavoliniidce, Cuvieria has a straight 

 conical shell, the hinder pointed half of which is generally caducous in 

 the adult. The genus is represented by existing species and by late 



Tertiary types. Lastly, in Cavo- 

 linia (Hyalcea) the shell (fig. 721) 

 is globular, the dorsal plate com- 

 paratively flat, and prolonged into 

 a hood, the ventral plate bulging 

 and convex, the aperture con- 

 tracted, with a lateral slit on each 

 side, and the hinder end with one 

 to three spines. The genus is represented by Miocene and Pliocene 

 forms as well as by existing species. 



Family 3. Cymbuliid^:. — This family includes only the recent 

 genus Cymbulia, which differs from all the other Thecosomatous 

 Pteropods in the possession of a straight, bilaterally symmetrical, 

 cartilaginous " pseudoconch," which is formed by the thickening of 

 the integument, and is not homologous with the shell of the ordinary 

 Pteropods. As the " shell " of Cymbulia is not capable of preserva- 



Fig. 721. — Cavolinia (Hyalced) Orbignyana. 

 Miocene Tertiary. 



