508 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



composed of the original cribiform plate, which constitutes their convex dorsal 

 surface, with an ingrowth of fasciculated tissue which closes over the axial canal 

 and gives to the ventral aspect a flattened surface with a median groove. This 

 ingrowth is most abundant toward the extremities, where also the meshes of the 

 reticulation are smaller, so that these parts have a more solid character than the 

 rest. 



We now begin to see that alternate inclination to one side and the other which 

 is so marked a feature in the articular extremities of the segments of the adult, 

 and we also notice that while these extremities are in close contact with one 

 another in the dorsal aspect, except where a small depression exists in each for 

 attachment of the articular ligament, the articular surfaces of the ventral face 

 slope away from each other so as to leave a considerable space for the lodgment 

 of the two muscular bundles which are now conspicuously interposed between 

 each pair of segments in the basal part of the arms, with certain exceptions. 

 These exceptions correspond to those which present themselves in the adult, though 

 the peculiarities of conformation which mark them are as yet but little pronounced. 

 Thus the adjacent extremities of the first and second brachials present nearly the 

 same aspect on their ventral as on their dorsal face, no beveling being seen on the 

 articular surfaces of either, and they are connected by ligament only, no muscular 

 fibers being here distinguishable. Again, the adjacent articular surfaces of the 

 third and fourth brachials come into yet closer contact, not even a connecting liga- 

 ment being interposed, and the line of their junction being transverse instead of 

 oblique. This prefigures the peculiar syzygial union which shows itself between 

 these and other pairs of segments in the adult. Toward the extremities of the 

 arms we find the segments even more cylindrical, except where lateral processes 

 are given off for the ai'ticulation of the pinnules, and the terminal faces are simply 

 apposed to each other transversely without either the alternate obliquity or the 

 beveled articular surfaces of the basal segments, and precisely the same rudimental 

 condition of the segments presents itself as that which has been already described 

 as showing itself at an earlier period in their basal segments. Thus we may trace 

 in one and the same arm several successive stages of development of the pieces of 

 its skeleton, the most advanced proximal segments showing adumbrations of their 

 adult peculiarities, though not as yet departing in any considerable degree from 

 their simple primitive type. 



Besides the brachials there are commonly found in the perisome of condensed 

 sarcode of the arms irregular branching spicules, which, like the orals, subsequently 

 undergo complete resorption so that no trace of them is discoverable in the adult. 



The sacculi, which are quite colorless in life, after death are immediately 

 and strongly stained by the pigment liberated from the perisome. 



The total number of segments in the fully developed pentacrinoid column 

 is subject to a considerable amount of variation. Thei'e may be as many as 24, 

 or as few as 16. The average may be considered as about 20. 



The total length of the fully grown pentacrinoid from the base of the column 

 to the extremities of the arms when these are folded together may be about 0.7 



