ECHINODISCUS. 531 



ECHINODISCUS. 



Echinodiscus Breyn., 1732, De Eclun. Schedias. (Leske emend.) 



The test in this genus is more depressed than in any other of the Scutel- 

 lidae ; the test is thin ; anterior edge rounded ; posterior edge truncated. 

 There are two lunules or cuts corresponding to the posterior ambulacra, Am- 

 bulacral petals small, well limited. Four genital pores. Lower surface flat ; 

 ambulacral furrows ramify but little towards the exterior edge. Anus nearer 

 the posterior edge than the actinostome. The greater part of tbe interior of 

 the test is occupied by a calcareous network rising into pillars for more than 

 half the distance between the edge and actinostome, leaving the central part 

 more or less covered by a delicate tracery of limestone cells, into which the 

 appendages of the alimentary canal are received. There are no pillars or par- 

 titions separating the buccal cavity from the alimentary canal. The aliment- 

 ary canal is slender, having the same general course as in other Scutellidae, 

 with the exception of a lateral fold, previous to its turning towards the anus 

 in the right anterior ambulacral space. The jaws, which are extremely flat, 

 articulate upon the auricle, which fits in a pit in the middle of the lower 

 surface of the jaw, and are not enclosed by them, as in other Scutellidae. 

 The spines are uniform in structure ; those of the upper surface are only 

 shorter and slightly clavate. 



Echinodiscus auritus 



Echinodiscus auritus Leske, 1778, Klein, Add. 



PI XI a .f. 9-13; PL XHP.f. is. 



This is the largest and most common species of the genus. The test 

 is depressed, rounded anteriorly ; the greatest width about half-way be- 

 tween the apex and posterior edge ; anterior extremity much narrower 

 than the broadly rounded and truncated posterior extremity. The vertex 

 is somewhat anterior, and corresponds with the apex. The petals are 

 small compared to the size of the test, and, judging from the specimens 

 examined, do not increase in size in proportion to the increase of the test. 

 The genital pores, four in number, are distinct, diverging posteriorly rather 

 more than in the other species of the genus. The petals are nearly of the 

 same size ; the odd petal is somewhat longer than the others. The petals 



