178 MANUAL OF THE MOLLUSCA. 



Shell, mucro chambered and siphuncled ; without lateral wings 

 or elongated beak. 



Helicertts, Dana. 



Example, H. Fugiensis. Only species known. 



Shell like a belemnite, half-inch in diameter ; guard thick, 

 sub-cylindrical, fibrous ; phragmocone slender, terminating in a 

 fusiform spiral nucleus. In slate rock, Cape Horn. 



Family YI.— Spirtjlib^. 



Shell entirely nacreous ; discoidal ; whorls separate, chambered 

 {polythalamous), with a ventral siphuncle. 



SPiRtTLA, Lam., 1801. 



Synonym, Lituus, Gray. 



Example, S. leevis (Grray). PI. I., Fig. 9. 



Body oblong, with minute terminal fins. Mantle supported 

 by a cervical and two ventral ridges and grooves. Arms with 

 six rows of very minute cups. Tentacles elongated. Funnel 

 Valved. 



Shell placed vertically in the posterior part of the body, with 

 the involute spire towards the ventral side. The last chamber 

 is not larger in proportion than the rest ; its margin is organically 

 connected ; it contains the ink-bag. 



The delicate shell of the spirula is scattered by thousands on 

 the shores of New Zealand ; it abounds on the Atlantic coasts, 

 and a few specimens are yearly brought by the Gulf- stream, 

 and strewed upon the shores of Devon and Cornwall. But the 

 animal is only known by a few fragments, and one perfect 

 specimen, obtained by Mr. Percy Earl on the coast of New 

 Zealand. 



Distribution, 3 species. All the warmer seas. 



Order II.— Tetrabranchiata. 



Animal creeping ; protected by an external shell. 



Head retractile within the mantle. Eyes pedunculated. Man- 

 dibles calcareous. Arms very numerous. Body attached to the 

 shell by adductor muscles, and by a continuous homy girdle. 

 Branchiae four. Funnel formed by the union of two lobes, which 

 do not form a complete tube. 



Shell external, camerated (polythalamous) and siphuncled ; the 

 inner layers and septa nacreous ; outer layers porcellanous.* 



* The Chinese carve a variety of patterns in the outer opaque layer of the nautilus 

 ehell, relieved by the pearly ground beneath. 



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