344 PROCEEDINGS OP TEE ACADEMY OF [1885. 



lateral rays, and four in the anterior ray, the number of inter- 

 radials at six to seven, with eighteen anal pieces. 



In this formula, the so-called first radials in four of the rays are 

 laterall}- separated from the rest by intervening interradial pieces, 

 a very uncommon but not altogether unprecedented occurrence 

 among Palreocrinoids. In Periechocrinus the radials are not 

 unfrequently found connected by their angles only, and some- 

 times, but exceptionally, one of them is altogether separated from 

 the rest by intervening interradial plates. Such a feature, thus 

 widely departing from the usual mode of occurrence, may in 

 certain cases become a fixed and constant character, but it must 

 not be overlooked, that by admitting the plates of this upper 

 ring as radials, it becomes imperative to extend the term radials 

 to every radial plate below, as each one of them is separated 

 from the preceding plate in a like manner. This would increase 

 the number of radials in Acrocrinus Wortheni to five (there was 

 evidently a small bifurcating piece filling the concavity of the 

 upper plate) in the lateral rays, and six in the anterior ra}', a 

 comparatively small number to what we must expect to find in 

 Acrocrinus Shumardi, if we adopt the above interpretation for 

 these plates. 



Through the kindness of Prof. Whitfield, we recently had an 

 opportunit}' to examine three specimens of the latter species from 

 the Museum of Natural History of New York, which have afforded 

 us additional information upon this interesting genus. 



Acrocrinus Shumardi is much larger than Acrocr. Wortheni, 

 and the calyx is composed of six to seven hundred pieces, while 

 in the latter it has less than one hundred. There are two large 

 basals ; two contiguous radials, the lower one small, pentagonal, 

 the other hexagonal with excavated upper side ; three interradial 

 pieces arranged as in the preceding species, the larger one resting 

 between both radials of adjoining rays, the two lower ones abut- 

 ting against the lower sloping sides of the second radials. The 

 above radials and interradials are distinctly separated from the 

 basals by a belt of small hexagonal pieces, which in position are 

 partly radial, partly interradial. They are arranged alternately 

 in rows, thoee of each successive series comparatively larger ; but, 

 while in A. Wortheni there are only four rings of from 12 to 14 

 pieces, Yandell's species has 14 to 20 rings, more or less, and 25 

 to 30 or more plates in each ring. Counting as before all plates 



