346 PEOF. p. M. DTJNCAlSr ON THE MORPHOLOGY 



plates of tlie amljulacra. The external dimensions of the pits along 

 tlie median lines are much smaller than and afford no indication 

 of their inner development. These pits pass deeply into the test 

 and enlarge within, especially actinally and abactinally, or in the 

 vertical direction, and less so laterally. 



In tlie interradial areas each pit leads to a compressed flask- 

 shaped cavity with a broad bottom. It does not perforate tlie 

 inner part of the test, but reaches inwards to within a very short 

 distance of the inner part of the edge of the plates, where there 

 is a layer of the usual reticulate, calcareous structure of the test. 



Each pit occupies space in the edges of the approximated plates, 

 and is surrounded, except superficially, by more or less project- 

 ing reticulate tissue, and is separated by it from the pits above 

 and below on the same line (Plate VIII. fig. 3). In some 

 instances the pits unite deeply with a neighbour ; so that if the 

 outline of the flat flask-shaped hollow were marked on the outside 

 of the test, to show its relation in point of size to the opening of 

 the pit, it would occupy an elliptical space three times the dimen- 

 sions of the orifice. As each plate is in relation with three 

 median pits, a considerable part of its edge is hollowed out, the 

 surface of the test being undermined. 



In the onedian line of tlie ambulacral areas the pits enlarge 

 as they pass inwards, are deep, flat flask-shaped, elongate verti- 

 cally, narrow from side to side, and frequently have a projection 

 on their floor. They occupy less space than the interradial series, 

 but undermine the plates close to their edges. It is perfectly 

 evident that the interradial, as well as the ambulacral pits of the 

 median lines are depressions on the ffices of the opposed edges of 

 adjoining plates (Plate VIII. figs. 3 & 8). 



The small pits between the tentaculiferous ambulacral plates 

 and the interradials pass inwards as cylindrical spaces ending in 

 cul de sacs ; some bend in their course, and others enlarge slightly 

 (Plate VIII. figs. 6 & 7). 



It is evident that in Sahnacis sulcata the pits, which are of 

 moderate dimensions at the surface of the test, occupy much 

 more space within it. They are lined with a continuation of the 

 ordinary derm of the outside of the test, and do not appear to 

 contain any special structures. They sometimes communicate, 

 within, with one another. 



On examining one of the interradial plates, the apex of its 

 angle, which is received between two corresponding interradials, 



