354 PKOF. p. M. DUNCAN ON TSE MOEPHOLOGY 



Amhlypneustes ovum. — A small specimen of this sj)ecies, in 

 wliicli the pits are mere depressions and barely pass inwards, 

 was examined. There are traces of the peculiar suturing to be 

 detected here and there ; but a more bulky convexity enters 

 an irregular concavity on the edges, in most parts. It is the 

 faintest expression of the very marked structures of Salmacis 

 sulcata. 



VI. BemarTcs on the Pits, Natural Grooves, and Sutures. 



The pits are more than simple depressions of the marginal 

 sutural lines ; and when fully developed, as in Salmacis sulcata 

 and Temnopleurus toreumaticus, they occupy space in the edges of 

 contiguous plates and portions of the test at the angles. They 

 commence, in the young form, as depressions on the sutural 

 margin ; and as the test increases in thickness the pit becomes 

 deeper, not only from the outward growth of the test, but also 

 from the inward growth and extension of the base of the cavity. 

 Each pit is a hollow in the approximated edges of two joining 

 plates, and some pits certainly communicate by their expanded 

 bases. The pits imdermine considerably, close to the edges of 

 the test in some instances, and are lined with a continuation of the 

 outer derm of the test. Loven found sphgeridia in those nearest 

 the peristome in the median ambulacral areas ; and I can testify 

 to their occurring as high as the sixth pit in the young form. 

 Elsewhere no special structures are in relation to the pits. 

 Similar developments are not known in any other subfamily of 

 the Echinoidea. 



The groovings and depressions along the line of the sutures, so 

 visible in Temnopleurus, and of much less significance in Salmacis, 

 increase with the age of the individual in the first-mentioned 

 genus ; and it is evident that they add in the first genus to the 

 extent o£ the superficies of the test. They may be broad or narrow, 

 deep or shallow, and their continuity may be interfered with by 

 vertical dissepiments or tubercles. They have an importance in 

 the economy of the animal; and they may be slight, and yet the 

 pits may be large. They are absolutely depressions between 

 ridges on which tubercles are placed above the normal level of the 

 plate, and which are ornamental elevations, as in Temnechinus, 

 which has no true pits, and Trigonocidaris. 



The sutures are composed of the ordinary reticulate transparent 

 calcareous tissue of the test. The knobs are more or less herai- 



