THE APODOUS HOLOTHURTANS 141 



three or four vertical rods more or less closely united by cross-bars. The 

 tables are usually rather small, the diameter of the disc ranging from 60 to 

 300 A 1 ; the spire is usually about equal to the diameter of the disc, but may 

 be much more. In the discs of some of the largest and most perfect tables 

 there may be as many as 70 perforations. The shallow, closed cups (Plate IX, 

 figs. 4, 5, 11, 12, 13) are the least common form of deposits, occurring only in 

 certain species of Caudina. They consist of a strongly concave plate through 

 which are four large, symmetrically placed, equal and similar perforations. A 

 conspicuous cross-shaped piece is so placed in the mouth of this shallow cup 

 that each arm of the cross is immediately over the middle portion of each per- 

 foration. The margin of the cup is usually more or less octagonal, and there 

 are often rounded knobs present at the end of each arm of the cross and half 

 way between. When fully and normally developed, these bodies are the most 

 symmetrical and ornamental deposits occurring among the Molpadiida?, but they 

 are often only partially developed, or they may be hypertrophied, especially in 

 the caudal region. The relative thickness of the cross-arms and the margin of 

 the cup, as compared with the diameter of the perforations, is very variable. 

 The perforated plates (Plates IX, figs. 3, 8; X, fig. 4; XII, figs. 2, 5-12, 

 14) exhibit the greatest diversity in form and appearance; they may be 

 nearly circular, or more or less triangular or square, or they may have one or 

 more conspicuous projections ("arms" or "handles"), which may be perforated 

 like the plate itself, or not. When a single handle is present, we have what are 

 called "racquet-shaped rods," and these are generally arranged in groups, 

 called "rosettes," with the large, perforated ends more or less overlying one an- 

 other at the center. The surface of perforated plates may be provided with stout 

 thorns or irregular projections, usually sharp, but it is commonly quite smooth. 

 The perforations may be few and very small or more numerous and larger; 

 commonly there are from 10-50, but there may be 60 or 70. The size of the plates 

 is quite variable, but their greatest diameter is commonly from 100-400 ^, 

 not including any handle. The fusiform bodies or rods (Plate XT, figs. 6-8, 13) 

 in their typical form are elongated, rounded particles, with more or less at- 

 tenuated but blunt ends, slightly flattened and enlarged at the middle and with 

 three or four small perforations in the center of the flattened part. They lie 

 at right angles to the long axis of the body and are often so abundant as to form 

 a more or less distinct layer; they are most likely to be present in the extreme 

 posterior part of the body. On the one hand they pass into the tables by the 

 development of a spire, the enlargement of the flattened area, and the disap- 

 pearance of the attenuate ends (Plate XI, fig. 12), and intermediate particles 

 in all stages of development are often found. On the other hand they may 

 become flattened and perforated at the ends, and thus pass by gradual tran- 

 sitions into the perforated plates ; such intermediate forms are very common. 



