510 MOLLUSCA phyltjm vi 



Altliougli this family is usually characterised by a small smooth shell, the essential 

 difference from the Dentaliidae is in the form of the foot. Typical forms of Gadulus 



appear in the Cretaceous ; the remaining 

 ^ J3 (-' Z) genera are Tertiaiy and Kecent. 



Entalina Monts. Shell I)entalium-Vik&, 

 largest at the aperture, thence taj^ering to 

 the apex ; strongly ribljed, and angular in 

 section near the apex. Miocene to Eecent. 

 Sifhonodentalium Sars (Pulsellum Stol. ; 

 Siplionentalis Sars). Shell an arcuate, 

 Fi"- ^*o. slightly tajjering tube, circular in section 



A,Ca(tulu.^ (PolysMdes) denticulatus Desh Cal- qj, nearly SO, and smooth externally. Apex 



caire Grossier ; Damery, near Bpernay. B, Cadulus it . , ,. . 



(DUrhhhs) hifissumtus Desii. Caicaire Grossier ; rather large, tyjiically slit into lobes, Ijut 



Gnjjnoii, near Paris. C, Cadulus omtlum. Fhil Tor- cmnptimes simnle Plioppne to ■Re.-mit 

 tonian ; Monte Gibbio. D, Cadulus olivi Scac. Tor- sometimes Simple. rilOCene tO Itecent. 



tonian ; Monte Gibbio, near Sassuolo, Italy. Cadulus Phil. {Gadus Desh. ; Gadila 



Gray ; Helonyx Stimp.) (Fig. 840, G, D). 

 Shell tubular, circular or oval in section, swollen near the middle or anteriorly, con- 

 tracting toward the aperture. Cretaceous to Recent. 



Tyjiical forms with simple anal orifice appear first in the Cretaceous, Dischides, 

 Jeffr. (Fig. 840, B), with two lateral slits, and Polyschides Pils. (Fig. 840, A), with 

 several notches, appear in the Eocene. All continue to the present time. 



Class 3. AMPHINEURA von Ihering.^ 



Aquatic, marine, hilaterally symmetrical moUusks, with the head partially or not 

 differentiated ; in form worm-like with a ventral groove or none, or oval, flattened, 

 with a foot adapted for creeping. Nervous system consisting of an oesophageal ring 

 with ganglia and four longitiidinal cords, two ventral and two lateral ; no cephalic 

 eyes, tentacles, or otocysts. Gills jMired or many, posterior or lateral ; mouth anterior, 

 usually with a radula ; anus posterior, median. External surface with a series of 

 eight shelly plates, or stiffened loith calcareous spicules. 



Order 1. APLACOPHORA von Ihering. 



Body vermiform, tvith a ventral groove, the skin elsewhere beset with calcareous 

 spicules ; no dorsal shelly plates in the adidt. 



This is a degenerate group, represented in the Recent fauna by about a 

 dozen genera belonging to two families — Chaetodermatidae and Neomeniidae. 

 Fossil remains are unknown. 



Order 2. POLYPLACOPHORA Blainville. Chitons. 



Amphineura protected by a dorsal series of eight shelly valves and an encircling 

 girdle ; with differentiated head, and a ventral sole or foot adapted to creeping ; gills 



^ Literature : Ihering, H. v. , Vergleichende Anatoiuie des Nervensystems mid Phylogenie der 

 Mollusken, 1877.— Dall, W. II., On the Genera of Chitons. Proc. U.S. Nat. Museum, 1881, vol. 

 iv.- — Jlnhrecht, A. A. IF., A Contribution to the Morphology of the Ainj)]iiueura. Quar. Journ. 

 Microscop. See, 1882, vol. xxii. [Bililiography, pp. 226, 227.]— Ruchebrune, A. T. de, Mono- 

 graphie des especes fossiles appartenant a la clas.se des Polyplaxiphores. Ann. Sci. Gcol. 1883, vol. 

 xiv. — Pruvot, O., Sur I'organisation de quelques Neomeniens des cotes de France. Arch. Zool. 

 Expi-r. et GencT. [2], 1891, vol. ix. [Bibliography, pp. 702, 7Q'3.]—Pilsbri/, H. A., Monograph of 

 the Polyplacophora. In Tryon and Pilsbry's Manual of Conchology, vols. xiv. and xv., 1892-93. 

 — Broili, F., Die Fauna der Pacliycardientuffe der Seiser Alp. II. Scaphopoden und Gastropoden, 

 Palaeoutogr. , 1907, vol. liv. 



