330 PROOEEDINOS OF THE ACADEMY OF [1881. 



a good character, but it must not be used to unite two groups, 

 otherwise distinct, in which the rim has been independently 

 developed, by modifications under similar conditions. 



Physetocrinus diflEers from Actinocrinus in the form and con- 

 struction of the vault, in having no anal tube ; in having pores 

 along the radial portions of the dome, and in possessing no hook- 

 like projections along the pinnules. 



Generic Diagnosis, — Body large, wider than high ; calyx semi- 

 ovate, much higher than the dome, which is depressed, convex, or 

 almost straight ; plates ornamented with beautiful striiB and with 

 a deep indentation at each corner of the plates. 



Basals three, large, truncate below, projecting out from the 

 body, and forming together a low cup. Primary radials 3 X 5, of 

 nearly equal size, the upper supporting 1X2X5 comparatively 

 large secondary radials, which all bifurcate and support the two 

 main divisions of the Yay. These extend out from the body, and 

 throw off on both sides lateral branches from each plate as in 

 Actinocrinus, which on becoming free turn into arms. The five 

 main rays, their divisions, and every branch, are separated by 

 small pieces, which toward the arm bases form deep sinuses, 

 extending deeply into the vault, and giving to the surface that 

 corrugated aspect which is so characteristic of this genus. 



Interradial plates more numerous than in Actinocrinus; they 

 consist of one in the first, and two in each succeeding series, grad- 

 ually decreasing in size. The upper plates, which are minute, join 

 with the plates of the vault, to form the depi-essions between the 

 main rays. There are three, five, or more interaxillary pieces, 

 which, connecting with the vault, form the sinuses between the 

 main division of the ray, while one or more interbrachial plates, 

 in a similar manner, separate the lateral branches. 



Anal area somewhat wider, and the sinus between the adjoining 

 rays a little deeper than in the other four interradial fields ; it 

 is composed of one large special plate in line with the first radials, 

 two smaller plates in the second, and generally three plates in the 

 succeeding series. 



Yault composed of rather delicate, extremely small tuberculous 

 pieces, which are disposed apparently without order or regu- 

 larity ; only the proximal dome plates being distinguishable. 

 There are small braces on the inner side of the floor, and the 

 entire vault is bent so as to form, in connection with the braces, in 



