NO. 1582. INFRABASALS IX THE FAMILY PENTACRINITID.E— CLARK. 675 



Fig. .5.— Section of calyx 

 OF Metacrinus superbus, 



SHOWIXG THE 1NFRABASAI..S 

 IN POSITION. 



if they are not seen, then it is a stem joint that is exposed to view, 

 and not the true dorsal surface of the basals. I believe that it was 

 this mistaking- of a stem joint for the dorsal surface of the basal which 

 prevented Dr. P. H. Carpenter from detecting the underbasals in the 

 recent Pentacrinitida^ during his work on the Challenger collection. 

 The other danger is that sometimes, when it 

 appears to be a moral certainty that " bottom " 

 has been reached, the infrabasals may be ob- 

 scured or entirely hidden by a stem joint which 

 is just beginning to form, and is composed of 

 almost invisible limy reticulations, not invis- 

 ible enough, however, to prevent the detec- 

 tion of the infrabasals through it. After a 

 stem joint has in its growth completely encir- 

 cled the central lumen, it is obvious enough; 

 but joints consisting of merely two or three ex- 

 ceedingly delicate lobes are very difficult to see. If Doctor Carpen- 

 ter's specimens were reexamined, I think it would be found that this 

 was also a source of error, as well as the mistaking of a stem joint for 

 the basals. I have found Jletacnm/s much easier to 

 handle than Isocrinus; but all my specimens of the 

 former are fresh and well preserved (taken in August, 

 IDOC)), while those of the latter are more than twenty 

 years old, so that possibly the difference may be due to 

 a difference in the state of preservation. 



The infrabasals of Metacrinus superbus are appar- 

 ently identical with those of M. roti/ndus, so that the 

 figiire published of the latter (fig, 3) will also serve to show the con- 

 ditions in the former. After a study of the dorsal surface of the 

 basals and infrabasals in place, by very delicate manipulation three 

 of the basals were removed, leaving the infrabasals in 

 place adhering to the other two. A sketch of the con- 

 ditions found was immediately made, and is reproduced 

 in fig. 5. The infrabasals themselves were then removed 

 and figs, 6, 7, and 8 were made from them. 



The infrabasals are long truncated-pyramidal plates, fig. 7.— ven- 

 equal in length to the entire height of the inner ends of 

 the basals. A side view of the five infrabasals together 

 is shown in fig, 6 : they form a truncated, dome-shaped, 

 sharply angular mass, somewhat broader than high, the 

 angles, of course, extending into the sutures between the basals. Near 

 the bottom (i, e., the dorsal side) the sides curve in somewhat abruptly 

 and the carination ceases, so that in a dorsal view (fig, 8) we get 

 no suggestion of it, the outer edges of the infrabasals then appearing 

 rounded. The sharp notch shown in the central infrabasal in fig. C 



Fig. 6.— Isolated 

 infrabasals 

 OF Metacrinus 

 superbus. 



tral view of 



isolated I N - 



frabasals of 

 Metacrinus 

 superbus. 



