346 PROCEEDINGS OF THE ACADEMY OF [1890. 



orals, of which the posterior one is larger and pushed in between 

 the four others, and of the so-called interradial dome plates, which 

 connect witli the interradials of the dorsal cup. Occasionally there 

 are outside the orals, a few other large pieces known as the radial 

 dome plates. In some species the orals are unrepresented, and the 

 whole ventral surface of the calyx is studded with interradial plates. 

 The mouth in none of the Camerata is exposed, but the ambulacra 

 wnth tightly closed food grooves are frequently visible. 



The inner floor of the ventral covering in most of the Actinocrin- 

 idae is deeply grooved so as to form well-defined galleries which lead 

 to the arm bases. Within these galleries is observed quite frequently, 

 but, so far as we know, only in BatoermiiS; Eretmocrinus, Dorycrimis, 

 Admocrinus, Teleioerinus and Physeiocrinus, a sort of second integ- 

 ument' parallel with the upper, which we took to be the disk. It 

 consists of rather irregular, ill-formed plates, arranged like those at 

 the upper surface. Each plate is connected with the corresponding 

 upper one by small surfaces or pillars, but the plates among them- 

 selves are not in contact laterally leaving open spaces or meshes be- 

 tween them. They constitute, as it appears, a kind of internal lining or 

 net-work, which extends from the first costals^ and first interradials 

 uninterruptedly to the margins of the orals, but no further. In some 

 specimens the inner plates are less distinct, and we find at the inner 

 floor small pillars or nodes, but nothing like plates. Within the 

 galleries the intervening spaces are occupied by the ambulacra. 

 The latter take the form of radiating tubes which run parallel with 

 the inner floor, and branch as often as there are bifurcations in the 

 calyx. Each tube is composed of four rows of plates, two above and 

 two below, the upper ones alternately arranged with a furrow along 

 the median line. The arrangement of the tubes and their mode of 

 branching are best observed in natural casts. Looking at such a 

 specimen, it seems indeed as if it could be scarcely possible that the 

 surface as it there appears, with the vault plates removed, the ambu- 

 lacra stretched out upon the upper face, resting within an integument 

 of irregular pieces, could represent anything but the disk as exposed 

 at the ventral surface of the Comatulae. 



1 For a more complete description of this integument see Revision of the Pal- 

 aeocrinoidea, Pt. Ill, p. 60. 



* The terms costals, distichals and palmars are explained in a paper of Dr. 

 Carpenter. (Ann. and Mag. Nat. Hist., July number, p. 15). 



