44 PANAMIC DEEP SEA ECHINI. 



.secondaries (Pis. 18, figs. 1, 3, 5 ; 20, figs. 1-',). The pairs of pores are 

 small and occupy a small corner of the ambulacra! plate (PI. 20, figs. 1-4), 

 while in S. Pailersoni the pores are large, distant, and occupy the whole 

 side of the ambulacral plate adjoining the interambulacral area (PI. 20, figs. 

 6, 7), and in S. liastigera the size of the pores is intermediate between the 

 two (PI. 20, fig. 8). In S. varispina the ambulacral pores are still smaller, 

 and take up but a fractional part of the ambulacral plate (PI. 21, figs. 5, 6). 

 Three of the actinal ambulacral tubercles are somewhat larger than the 

 others (PI. 20, figs. 1-4), showing a Hemicidaris-like arrangement, while the 

 others are uniform in size, decreasing only in dimensions at the last plates 

 near the abactinal system. The larger tubercles are often crenulated 

 (PI. 19, fig. 1). 



There is one important feature in the structure of the ambulacra which 

 gives us the measure of the amount of resorption which has taken place at 

 the junction of the ambulacra with the actinal system. The resorption 

 of the actinal plates does not seem to be sufficient to absorb wholly any 

 of the ambulacral plates, though in a specimen of Salenia miliaria of 13 mm. 

 the first ambulacral plate of the right zone of the odd anterior ambulacrum, 



m 



^J 



4 



13 mm. 



Fig. 71. Salenia miliaris. 



Fig. 71, has nearly disappeared (PI. 20, fig. S), and there is no trace of the 

 interambulacral primordial plates in any of the young of Salenia I have 

 examined except in a very young S. varispina of less than 2 mm. This corre- 

 sponds to what takes place in the Cidarida3, in which the primordial plates 

 have been observed only in the very youngest stages. 



The stress of the coronal ambulacral plates against the edge of the 

 actinal system produces an asymmetry (PI. 20, fig. ..'), as in Dermato- 

 diadema and other Echinoids in which there are but few calcareous plates 

 sloughed off onto the actinal system from the adjoining coronal plates, a 

 slight thickening of the actinal lips (PI. 20, fig. 3), and retrogression to 



