62 Prof. P. M. Duncan on the Salenidfe. 



apology for proceeding to analyze and compare, but express 

 my hearty obligation to Prof. R. Tate for his giving me, in 

 common with the Fellows of the Geological Society, the oppor- 

 tunity of studying his interesting specimens. There are four 

 specimens of the species, the smallest being -p\ inch in 

 breadth and rather more than -^ in height ; and there are two 

 others which form, with the largest one (the type), a series as 

 regards increasing size. All present a community of form, 

 characterized by the relatively small apical system, the eccen- 

 tric irregularly pentagonal vent, the large numerous interam- 

 bulacral tubercles, and the ambulacra forming comparatively 

 narrow zones having four vertical rows of small secondary 

 tubercles. So far as growth is concerned, the specific charac- 

 ters so ably distinguished by Prof, Tate are well shown in 

 all the specimens ; and it would appear that there is a tendency 

 with age to increase in height beyond the average breadth. 

 Of course the number of the primary interambulacral tubercles 

 increases with age, and the number and relative size of the 

 ambulacral secondaries and pores also. 



The small secondary tubercles of the ambulacra are more 

 worthy of that term than that of large granules ; for a careful 

 examination shows each of them to have a non-crenulate mame- 

 lon with a small rounded imperforate boss. There is a 

 marked distinction, during the growth, between the size of 

 these secondary tubercles — a few remaining larger than the 

 others at the actinostomial end of the narrow petaloid ambu- 

 lacra. The outer vertical rows have these small tubercles 

 larger and taller than the inner two rows ; and there is an 

 alternate arrangement of the four rows, the larger and external 

 tubercles not being opposite to each other. The minute 

 granulations of the ambulacra situated between and around 

 the bases of the secondary tubercles resemble the pedicellaria- 

 carriers of the recent types, and doubtless had this function. 

 They are numerous and yet not crowded. 



The pores, in pairs, are oblique, small, round ; and the pairs 

 are separated by a rounded ridge which starts from the outer 

 side of each mamelon of every outer ambulacral tubercle ; and 

 there is a slight ridge between each pore in the pair. The 

 pairs of pores in the smallest specimen number twenty in each 

 zone, and from thirty to thirty-five in the largest. Five pairs 

 of pores can be counted in relation to the large plates of the 

 interambulacra, which carry the two largest tubercles above 

 the ambitus. There is but one pair of pores to each ambula- 

 cral plate. 



Some of the pores near the actinostome have a circular rim 

 to the pair. 



