'^40 PALEONTOLOGY OF NEW-YORK. 



that the plates covering it were similar to those in the following species. The anteal or basal 

 pectinated apertures present the characters noticed in the general review of these parts, having 

 the lower one of the pair sub-triangular, with apex placed towards the base, while the higher 

 one is nearly oval or sub-reniform. In the higher pairs, this relative position is reversed ; and 

 in the pair placed nearly horizontal, the one slightly higher than the other is the triangular 

 one. The triangular apertures have a thinner border, and are larger and less deep than the oval 

 ones, which gradually expand outwards, and the border is labiate. The transverse bars which 

 can be counted in the higher, or triangular ones of the left and right sides, and the base or anteal 

 side, are respectively 9, 13 and 16 ; while in the lower or oval ones there are a less number, 

 apparently having the same gradation, but not distinctly ascertained. If a line be drawn through 

 the specimen in a direction from the anteal to theposteal side, the mouth and associated organs 

 are on the left side ; the bases of two of the arms arch over these organs, including between 

 them the pair of pectinated apertures on the left side : one of these arms is dichotomous below, 

 or near the extremity. On the right side diverge three arms, two of which, arching over the 

 right pectinated apertures, are dichotomous near their extremities ; while one of the simple 

 arms goes to the right of the anteal pectinated apertures, and to the left of the ovarian aperture. 

 The bifurcation of the arms may not be of specific importance, but following or coincident 

 with some other modifications. The arms are, as described, composed of a double series of 

 plates laterally in contact, and resting in a shallow groove in the plates of the body. Each 

 series of joints is similar, and composed of a large and small joint alternating with each 

 other : the large joint has its inner margin convex, and longer than the outer margin ; the 

 smaller plate has a long external margin, while the inner margin is extremely narrow, and not 

 projecting as far as the margins of the adjoining plates. The groove between the rows of the 

 plates of the arms is excavated from the plates of both series, but not entirely through them, 

 as they still join by their margins below. The sinus is made by the narrow margin or angle of 

 the smaller plate, which is intercalated between each two of the larger ones, but does not pro- 

 ject as far into the centre as those. At the bottom of this sinus, the tentacula take their origin. 

 This groove, in the living condition of the animal, was undoubtedly occupied by some organ 

 diverging from the centre, extending to the extremity of the arms, and finally diverging into 

 the bottom of each sinus. From some remaining evidences, it seems that this organ was com- 

 posed of minute articulating joints, probably covered by a fleshy envelope, and possessed the 

 power of keeping up a communication from the centre along the line of the arms, and to each 

 individual tentaculum ; so that from the most remote points of these, sensation was communi- 

 cated at once to the centre. The similarity of this groove to the canal or avenue on the under 

 side of starfishes, is too obvious to escape attention. The little porous tubercle near the mouth 

 also strongly reminds one of the madreporiform tubercle in Asterias and other Echinoderms. 



Fio-. 1. The posterior side, showing the ovarian aperture destitute of plates closing it, the arms 

 on each side arching over it, and the pectinated apertures on the right side, with 

 the margin of those on the left, between which and the aperture is a dichotomous 

 arm. 



