388 



ECHINOZOA. 



included in this group the genus Echinobrissus is one of the most widely 

 distributed, species of the genus being very abundant in the Jurassic 

 rocks, but extending also into the Cretaceous. In this genus (fig. 263), 

 the test is concave below, and the anus is placed in a dorsal sulcus. 

 Other well-known genera are Nucleolites (Chalk to Recent), Pygurits 

 (Jurassic and Cretaceous), Clypeus (Jurassic), Pygaulus (Cretaceous), 

 Cassiduliis (Cretaceous and Tertiary), Echinanthus (Cretaceous and 

 Tertiary), and the Tertiary and Recent Catopygus and Echinolamftas. 



Family 5. Holasteridce. — This family comprises ovoid Urchins, 

 with simple ambulacra, and a generally very gibbous test. The 

 apical disc is usually drawn out in an antero-posterior direction, the 

 elongation being sometimes so great that the two posterior ambulacra 



Fig. 264. — Collyrites {Dysaster) Ezidesi, viewed from above, from one side, and from below. 



Jurassic. 



are widely separated from the three anterior ones on the summit of 

 the shell (fig. 264). The peristome is excentric, generally oblique 

 or bilabiate ; and the anus is inframarginal or marginal. The mem- 

 bers of this family are mostly Jurassic and Cretaceous, but a number 

 of recent forms are known to exist. 



In one group of this family (the CollyritidcE of D'Orbigny, or the 

 Dysasteridoz of other writers), the apical disc is greatly elongated, and 



the narrow ambulacral areas thus become 

 more or less " disjunct," the two hinder 

 ambulacra meeting superiorly at one end 

 of the disc, while the three anterior ambu- 

 lacra meet at the other end of the same. 

 The principal genus of this group is Col- 

 lyrites itself (figs. 264 and 265), in which 

 the test is oval or somewhat heart-shaped, 

 the mouth being inferior and excentric, and 

 the anus being supramarginal. The ele- 

 ments of the apical disc are detached from 

 one another, the anterior portion contain- 

 ing four perforated genital plates and three 

 ocular plates, while the posterior portion, 

 connected with the preceding by a narrow 

 series of supernumerary plates, contains 

 the two other ocular plates. The genus is 

 represented by numerous species in the 

 Jurassic and Cretaceous rocks. Closely 

 allied to the preceding, and with a similar geological range, is the genus 

 Dysaster. 



Fig. 265. — Upper surface of the 

 test of Collyrites elliptica, showing 

 long apical disc and disjunct ambu- 

 lacra. Jurassic. (After Zittel.) 



