42 SMITHSONIAN INSTITUTION 



indicate their position; but the branching ambulacra in the tegmen show it 

 perfectly. These, corresponding in position to brachials either actual or poten- 

 tial on the dorsal side, have a main series of covering plates leading to the 

 arm-openings, and from the sides of this smaller series branch off and lead 

 to two or more pores at either side of the arm-bases. The branching ambulacra 

 are well shown in Wachsmuth and Springer's figure of the tegmen of Marsipo- 

 crinus radiatns (see N. A. Crin. Cam., pi. 8, fig. 15), reproduced by Bather in 

 Lankester's Zoology (p. 124, fig. 32). The exact relation of the smaller 

 branches to the plates, and to the pores themselves, is better shown in text- 

 figure 19, diagrammatized from one of the Tennessee specimens figured on 

 Plate IX, figure 6. The first lateral branches and their pores are seen to belong 

 to the secundibrachs, with one large pinnule ossicle incorporated, and are 

 followed by those from the interlocking brachials of the arm. All that is 

 needed further is to show the pinnules themselves which we now have per- 



19 



Figs. 17-19 

 Fig. 17, Batocrinus clypeatus. Transverse section at arm-openings, plates separated 

 at sutures ; the passages from the so-called " respiratory pores " connecting with the food 

 canals. X f- 18, Cyphocritms gorbyi. An interradius with adjacent arm-bases showing 

 pores in plates corresponding in position to pinnule ossicles. X &• 19, Marsipocrinus 

 striatus(?). Diagram of one ray of specimen figured on PI. IX, fig. 6, showing position 

 of pinnule openings and ambulacra leading to them. X |. 



fectly preserved, both brachial and interbrachial, in the beautiful specimen of 

 M. tennesseensis figured on Plate IX, figures 50, b. 



An excellent example of secundibrach pinnules is seen in Platycrinus 

 huntsvillce, where they are often very conspicuous, and stand out distinctly 

 larger than those upon the arms (they are well shown in N. A. Crin. Cam., 

 pi. 73, figs, jb and 9). In this species, as well as in the Marsipocrinus, it is 

 the first secundibrach which bears the pinnule, these being forms in which the 

 primibrach series is reduced to a single plate — the axillary. 



In the foregoing text-figures 17-19 are shown various stages of develop- 

 ment in the incorporation of pinnules, and also the different methods by which 

 their ambulacra empty into the main ambulacra : 



The first stage is that in which an arm ossicle is incorporated in the 

 dorsal cup without incorporation of its pinnule {Batocrinus, fig. 17). In this 



