244 LECTURES ON MOLLTJSCA. 



Family HYALiEiDyE. 



The Hyalceas are protected by a globular shell, consisting of a dorsal 

 and ventral plate, (as in the Palliobranchs,) united at the tail. The 

 two fins are retractile into the shell, and unite round the mouth. 

 There are two tentacular processes behind, passing through side-slits 

 in the shell, showing a resemblance to Cymodocea. In Diacria these 

 processes are very small and inclosed, while the tail is produced. 

 Cleodora has a glassy, pyramidal shell, of three flat sides, each ending 

 in a spike. In Balantium the shell is funnel-shaped, not spiked. 

 Creseis has a very slender, pointed, circular funnel. In Cuvieria, the 

 shell is swollen at the base like an urn, generally with the point trun- 

 cated. The point remains permanent inthe Vaginella of the Bordeaux 

 beds. 



Family CoxuLAEJADiE. 



The great carboniferous fossil Conularia was probably nearly related 

 to Cleodora and C?~eseis, but as its relations are not clear, it is kept in 

 a separate family. The shell is four-sided, and very beautifully 

 striated across. In the Devonian form Coleoprion, the angles are 

 rounded-off. The Silurian Theca has a shell like an elongated 

 Cleodora, without spikes. Pterotheca has wing-like projections at the 

 sides. 



Family LiMACiNiDiE. {Spiral Pteropods.) 



The tiny shells of Spinalis are spiral, with the point either raised 

 or depressed. Between the fins is the rudiment of a foot bearing an 

 operculum. These creatures furnish the nearest approach to the larval 

 Gasteropods. In Limacina the mouth is round, and there is no oper- 

 culum. The shells of this family may be known from the Macgilli- 

 vrayids, by being always reversed. 



Family Cymbuliad^e. (Glass- Slippers.) 



The lovely Cymbidia inhabits an elegantly-cut cartilaginous shell, 

 foreshadowing the Argonaut, the wings flapping on each side, as the 

 sails of that Cuttle were formerly supposed to act. The lingual teeth 

 in this genus, and in Eurybia, (which has a cup-shaped boat, and 

 tentacles,) are arranged in three series.. Eurybia similarly foreshad- 

 ows Bursatella among the Opisthobranchs. Tiedemannia is like a 

 Cymbidia without the glass-slipper, forming a transition to the first 

 order ; while the delicate little Psyche seems no more than a minute, 

 transparent globe, wafted over the banks of Newfoundland by its 

 spreading wings. And so end the higher groups of Molluscous 

 Animals. 



CLASS LAMELLIBRANCHIATA, 



(or Plate-gilled Bivalves.) 



The remaining classes of mollusks present us with a very different 

 type of organization ; inferior, indeed, to the head-bearing tribes, and 



