80 



PALEONTOLOGY. 





Fig. 22. 

 Hyalcva tridentata. 



the delicacy and transparency of its texture. It deviates 

 least from the ordinary form of the spiral univalve in Spi- 

 nalis and some allied fossils. In Limacina the turns of the 



shell are reduced to one whorl and 

 a half. In Hyalwa (fig. 22) the 

 shell resembles a bivalve, in which 

 the two valves have been cemented 

 together along the hinge k, leaving 

 a narrow fissure in front and at the 

 sides. The ventral valve, g, is most 

 convex, the dorsal one, /, most produced, overhanging the fis- 

 sure-like opening, i, through which the head and swimming- 

 lobes are protruded. In Cleodora the shell is narrowed and 

 lengthened out, the two plates being united together along the 

 sides, so as to leave only an anterior aperture. 



Fossil shells of both Hyalcea and Cleodora are found in the 

 newer tertiary of Italy, with Vaginella (fig. 28, 12), a form 



allied to Cuvieria (fig. 23). But 

 1 the occurrence of Pteropoda in 

 the older rocks is attended with 

 considerable obscurity. The Eu- 

 ompthali (fig. 26, 4), which charac- 

 terise those rocks, have multispiral calcareous opercula, like the 

 recent Cyclostrema (=Adeorhis). The genus Maclurea (fig. 26, 9), 

 which has been regarded as a " left-handed" Euompthalus, is pro- 

 bably very different ; it has a thick shelly operculum, sinistrally 

 spiral, and furnished with an internal process, as the Nerites 

 are; the spire is sunk and concealed, whilst the whorls are 

 exposed on the flattened under-side; it occurs in the older 

 Silurian rocks of Scotland and North America, In Euomp>hilus 

 rugosus, from Illinois coal-beds, all the volutions are exposed 

 in the wide and shallow umbilicus. Eccidiomphalus is like 

 an incompletely convoluted Euomphalus ; Maclurea is like 

 Euomplialus with a depressed spire; the shells called Theca 



y 





Fig. 23. 

 Cuvieria columnella. 



