386 



ECHINOZOA. 



The position of the family Echi?ioconidce has been rendered doubt- 

 ful by the researches of Professor Martin Duncan, who has shown 

 that the type-genus, Echinoconus, usually regarded as possessing teeth 

 and " auricles," is really destitute of both these structures. In ac- 

 cordance with this, Dr Duncan would remove the genus Echinoconus 

 to the family of the Cassidulidce. The forms included in the family 



Fig. 259. — Discoidea cylindrica. The right-hand figure shows the summit of the shell, with 

 the genital disc. The left-hand figure shows the base of the shell, on which are situated both the 

 mouth and anus. Cretaceous. 



Echinoconidce are almost wholly confined to the Jurassic and Creta- 

 ceous rocks, so far as at present known ; but a living species of 

 Pygaster has been detected, and this genus, at any rate, must have 

 representatives in the Tertiary rocks. 



The genus Echinoconus ( = Galerites) is exclusively confined to the 

 Cretaceous rocks. E. vulgaris and E. conicus ( = Galeritcs albogalerus) 

 being very common species in the White Chalk. 

 The test in this genus (fig. 239) is rounded or 

 subpentagonal, often conical, with a nearly flat 

 base. The anus is infra-marginal or nearly mar- 

 ginal. Discoidea (fig. 259) is also confined to 

 the Cretaceous rocks, and is nearly related to the 

 preceding, but the margins of the interambulacral 

 areas in the interior of the test are strengthened 

 by longitudinal thickenings or ribs. Holectypus, 

 again, is closely allied to Discoidea, but the test 

 is depressed (fig. 260), and there are no internal 

 ribs. The species of this genus are Jurassic and 

 Cretaceous. Lastly, in the genus Pygaster (fig. 

 261), the anal aperture is of great size, oblong 

 or pyriform in shape, and situated on the superior 

 aspect of the shell. All the five genital plates 

 are present, and are perforated for the generative ducts. The species 

 of this genus are mostly Jurassic and Cretaceous, but a recent species is 

 known to exist. 



Fig. 260. — Test of Holec- 

 typus hemisphericus, viewed 

 from above — Jurassic. (After 

 Edward Forbes.) 



Family 4. Cassidulidce. — This family comprises irregular Urchins 

 in which the mouth is invariably edentulous, and is placed centrally 

 or subcentrally on the inferior surface, while the anus is excentric, 

 and is generally placed on the upper surface (fig. 262). The test 

 is generally oval or elliptical, and the ambulacral zones are approxi- 



