THE JTAMILT ABBACIABiE. 33 



The Pits along the Median Suture. — As many as five deep pits 

 may be seen in some species of Gcelopleurus along the median 

 line of the ambulacra at the spots where the transverse sutures 

 between the consecutive compound plates unite at the median 

 line with the corresponding angular process of the neighbouring 

 plate of the other zone (PL I. fig. 7). The first pit is close 

 above the concave margin of the ambulacrum at the . peristome, 

 being separated from it by a nai'row ridge, or it may be at the 

 margin aud even turned slightly inwards. 



The pits form a zigzag up the median suture, and it is evident 

 that each one was arched above and that it had a flat broad base. 

 The sutures may sometimes be seen passing along the hollowed- 

 out deeply-seated surface. 



The growth of the test was attended with a downward move- 

 ment of the whole of the peristomial part, as well as of the other 

 parts of the ambulacra, accompanied, apparently, by absorption 

 of the oldest tubercles, pores, and pits. 



The Ambulacra near the Radial Plates.- — This part of an am- 

 bulacrum extends from the radial plate to the first large tubercle- 

 bearing compound plate. It is narrow in comparison with the 

 interradia on either side and with the part of the ambulacrum at 

 the ambitus ; but the poriferous zones are straight and the pairs 

 of pores are increasingly wide apart and the pores are very large, 

 except close to the radial plate (Pi. I. figs. 4 and 5). The 

 pairs are not so distant as are the pores of the same pair, and 

 the direction of the pores of each pair is oblique from within, 

 outwards and upwards. The part of the plates between the pores 

 of each pair is broad, and it is sometimes larger in this region 

 than in that of the great tubercles. 



The interporiferous areas are crowded with plates of very 

 diverse shapes and dimensions, so as to resemble a broken and 

 confused mosaic. The apparently disorderly arrangement is 

 greatest midway between the radial plate and the first great 

 tubercle, and great and little plates, some with, and others with- 

 out, small tubercles, are jumbled together as if there were not 

 room for them all. Some of the plates are largest at their pori- 

 ferous part ; others are larger, and their interporiferous expansion 

 extends far over the median line and also adorally and aborally, 

 so as to exclude the plates immediately above and below from the 

 median line. Or the expansion may not be so great, and there may 

 be some plates which reach the median line in succession, but 



LINN. JOUBN. — ZOOLOay, VOL. XIX. 3 



