INVERTEBRATE ANIMALS. 41 



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The Mark ^ indicates specimens or other illustrations exhibited in the Cases. 



Group 121. Family BELEROPHONTID^ (MacCoy). 

 Estimated number of species 70, known only as fossils, 

 chiefly Silurian and Carboniferous. The position of the 

 group is uncertain, the animal being quite unknown 

 except through the remains of its shell. 



Class CEPHALOPODA. 



xs<paKyj, the head ; ttouc, a foot. 

 Organs of progression encircling the head. 



Order TETRABRANCHIATA (Owen). Gills 

 4 ; arms many, without suckers, resembling tentacles. 

 The shells in this very extensive order, of which only a 

 single living form remains (the Nautilus), are all 

 chambered, i.e., divided transversely into compartments, 

 in the outermost of which the animal resides, keeping up 

 a communication with the previously inhabited chambers 

 by means of a siphuncle or tube. 



Family AMMONITID^ (Owen). "AMMON, 

 a title of Jupiter when represented as having the horns 

 of a ram. 



Group 122.— Genus HAMITES (Parkinson); and alHes. 

 Hamus, a hook. This group includes the genera of the 

 Ammonite family in which the tube of the shell is 

 nearly straight or sharply bent near the extremity; also 

 genus Turrilites, in which the chambered tube forms a 

 beautifully spiral shell. Estimated number of species 

 140, all known as fossils of the Cretaceous system. 



Group 123.— Genus AMMONITES (Bruguiere). Sutures 

 of the shell more or less ramified. Estimated number 

 of species 530, occurring from the Trias to the Chalk. 

 IT Specimens of Ammonites from the Wealden, Oolite, 

 Lias, Chalk, and Green-sand formations. 

 e 



