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According to the data now on record, respecting the past 

 history of the genus Amphidetus, the latter made its first appear- 

 ance on the European continent, as early as the eocene period 

 of the tertiary era. The living Aaierican species, the object of 

 the present communication, I propose to call 



Amphidetus KiJRTzii Girard. 



It is one of the largest species of the genus, the specimens 

 in my possession measuring more than two inches and a 

 half in length, being nearly as broad as long across the 

 middle of the anterior half of the body. Its most remarkable 

 feature consists in the great development of the ambulacral 

 star, which, at the same time, is very much depressed. The 

 ambulacra are situated each in a broadly open groove, the 

 anterior and odd one deeper than the others, its groove ex- 

 tending to the inferior surface. Interambulacral spaces above 

 convex and having a swollen appearance. Posterior ambu- 

 lacra conical and tapering backwards for more than half of 

 their greatest width. The anterior ambulacra have nearly the 

 same width throughout their whole extent, being narrower, how- 

 ever, than the former at their base. They are slightly bent 

 backwards along the sides, although their general direction is 

 forwards. The odd ambulacrum is composed of exceedingly 

 minute pores, which are, however, very conspicuous on account 

 of their being very close to each other, and arranged in a double 

 row. The series are parallel and nearly equidistant, but con- 

 nected near the summit of the ideal axis of the animal by a 

 gentle hemispherical curve in immediate advance of the four 

 openings for the organs of generation, which are likewise very 

 conspicuous. 



The narrow and smooth tape-shaped zone which surrounds 

 this ambulacrum, constitutes an elongated pentagon, the poste- 

 rior angle of which is acute and reaches the summit of the pos- 

 terior and odd interambulacral space. Its sides are parallel, 

 whilst it is irregularly rounded anteriorly. The area between 

 the series of pores and the triangular region behind the ambu- 

 lacrum, is densely covered with minute tubercles. The so-called 

 smooth zone itself is covered with still smaller tubercles, which 

 escape detection by magnifying glasses of one and even two 

 diameters. 



