342 RHYxNCHUPYGUS. 



(CASSIDULUS.) Rhtnchoptgus. 



Rhynchopygus D'Orbig. 1855. Pal. FrariQ. 



Lamarck's genus Cassidulus, as established in 1801, contains in it two dis- 

 tinct types : Cassidulus lapis caneri ; the species from the West Indies and 

 Cassidulus Mannimi. This has been separated as a distinct genus, Rhyn- 

 chopygus, by Desor, with which the recent species of the genus must also be 

 associated, as has been proposed by Liitken. After an examination of a fine 

 series of fossil species of the genus Cassidulus. 1 have been led to modify the 

 opinion 1 had expressed of the affinities of our recent species, and to return to 

 the view originally proposed by Desor and Liitken of uniting the recent West 

 Indian species to the genus Rhynchopygus, regarding this, however, only as 

 a subdivision of Cassidulus ; for, notwithstanding the transverse position of the 

 anus, covered in part by a projection of the test, and the absence of a promi- 

 nent anal furrow, we find in the species of the genus Cassidulus indica- 

 tions thai these characters are not of primary importance, though in the 

 present state of our knowledge they may serve to subdivide the genus 

 conveniently. The separation I had made in the Preliminary Report as 

 Kliyncliolampas of our recent Atlantic and Pacific species is not warranted ; 

 a series of C. Mannimi shows that the smooth band of the actinal surface 

 and the position of the large tubercles are subject to great variation, reducing 

 the distinctions to merely specific characters. The genus Rhynchopygus 

 includes, as here limited, species with a thin test, well developed petaloid 

 ambulacra; four genital openings: a transverse anal system placed above the 

 edge of the test, covered by a projection of the overhanging interambulacral 

 part of the test; a slightly marked anal furrow. The lower surface is slightly 

 concave, sloping towards the outer edge ; mouth eccentric anteriorly, bourre- 

 lets and phyllodes highly developed; a broad bare band extending from one 

 extremity of the test to the other. Tubercles of the upper part of the test 

 uniform, small, resembling Clypeastroid tubercles; tubercles of the lower part 

 large, deeply sunken as in Spatangoids; adjoining bare band, they gradually 

 diminish in size towards the edge of the test. Apical system more central 

 than mouth, eccentric anteriorly. 



