He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in Limacinidae.
The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphonof Cephalopods which is formed from epipodia.
The next six families include the animals formerly known as Gymnosomatous Pteropods, characterized by the absence of mantle and shell, the reduction of the ventral surface of the foot, and the parapodial fins at the anterior end of the body.
In most other Mollusca (Anisopleurous Gastropods, Pteropods and Conchifera) there is a want of such continuity; the primitive shell-sac contributes no factor to the permanent shell, or only a very minute FIG.
When the proportion of calcium carbonate in the blue mud is considerable there results a calcareous ooze, which when found on the continental slope and in enclosed seas is largely composed of remains of deep-sea corals and bottom-living foraminif era, pelagic organisms including pteropods being less frequently represented.
Murray and Renard define globigerina ooze as containing at least 30% of calcium carbonate, in which the remains of pelagic (not benthonic) foraminifera predominate and in which remains of pelagic mollusca such as pteropods and heteropods, ostracodes and also coccoliths (minute calcareous algae) may also occur.
Pteropod ooze is merely a local variety of globigerina ooze in which the comparatively large but very delicate spindleshaped shells of pteropods happen to abound.
,Among the known fossils are vermes, crustacea and probably brachiopods and pteropods The character of the sediments of the Proterozoic is such as to show that mature weathering affected the older rocks before their material was worked over into the Proterozoic formations.
Some, like many Cephalopods and the Pteropods, are pelagic or free-swimming; others creep or lie on the sea bottom.
Each of the principal groups of true mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenella, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyo- tithes, Salterella); Cephalopods (Orthoceras, Cystoceras).
The Pteropods were well represented by Conularia and Bellerophon.