1881.] NATURAL SCIENCES OP PHILADELPHIA. 199 



hook ; but in all cases they lie so close together side by side, that 

 it appears as if the pinnules were laterally- attached. In Activo- 

 crinus and Strotocrinus each pinnule is furnislied near the middle 

 of its outer surface with a tooth-like spine Avhich curves abruptly 

 upwards ; these spines are short and obtuse near the arms, but 

 gradually increase in length toward the tips of the pinnules. As 

 a rule the pinnules are deeply grooved on their inner surface, and 

 in perfect specimens the grooves are covered by a double series of 

 very minute pieces, though owing to defective preservation this 

 covering is rarely observed. In young specimens, while the arms 

 are composed of a single series of joints, the pinnules are not in 

 contact, and are onl}^ given off from the alternate joints, but when 

 the alternate arm joints meet b}^ lateral growth, and the pinnules 

 attain their full size they become gradually connected. D'Orbigny's 

 genus EdwardaociHnus was founded upon a young Flatycrinus, 

 whose arms and pinnules were in their transition state. 



T. Internal Cavity. 



The construction of the interior of the bod}' of all Paleozoic 

 Crinoids is best known in the Sphctroidocrinidte, among which, 

 specimens preserving some parts of the delicate organs have occa- 

 sionally been found. 



The inner surface of the vault is ofcen deeply grooved toward 

 the brachial zone, producing corresponding elevations outwardly 

 on the test. There are generally five large grooves, each branching 

 into two smaller ones, the former corresponding to the five rays, 

 the latter to their main divisions. This kind of vault is found 

 most frequently among the Rhodocrinida?. Among the Actino- 

 crinidae external ridges are rarely observed, but in their place 

 the vault within is strengthened with bars or braces radiating 

 from near, but not joining at the centre. The braces widen 

 toward the arm bases, where they fold over to form regular tubes, 

 corresponding with the natural grooves in the vault just 

 described, and they branch as those do. In genera in which the 

 rays are extended into free appendages, and in which but five 

 ambulacral canals pass out from the vault proper (Flatycrinus, 

 Steganocrinus, etc.), the grooves are deep and in some cases were 

 evidently closed and formed into tunnels, leaving, however, in 

 either case beneath the median portion of the dome and in front 

 of the anus, a space which is occupied by narrow grooves, meeting 



