THE FAMILY AEBACIAB^. 47 



pits and intervening projections ; but, as A. Agassiz has noticed, 

 there is no structure at that spot like that of the Temnopleuridae. 



VII. The Interradial JPlates and tlieir Sutures. 



The interradial plates which succeed the small ones imme- 

 diately around the apical system are remarkably high and broad, 

 and thus they differ from those of all the fossil species. The 

 slight development of secondary tubercles between the great 

 primary tubercles of the interradia and the poriferous zone of 

 the ambulacra, appears to prevent that breadth and obliquity of 

 the plates which is seen in the fossil forms. The great tubercles 

 are of the same shape as those of the ambulacra ; and when the 

 inside of the test is examined, the surface beneath them is seen 

 to be hollowed. 



The suturing of the plates together is excessively close ; and in 

 the specimen which one of us has prepared for the National 

 Collection, aad which had been kept in alcohol, the test often 

 broke across the plates themselves instead of at the sutural 

 lines. 



The transverse sutures between the interradial plates sej)arated 

 much more readily than those of the vertical or median zigzag line. 

 Any separation along these median sutures is hard to get, and 

 there is an evident and very iuteresting reason. 



On the adoral suture of every iuterradial coronal plate there is 

 a series of knobs, and these prominences of the tissue of the 

 plates are either hemispherical or elongated. There are also 

 small pits placed here and there, between the knobs, aad the 

 whole surface of the sutural face is somewhat swollen (PI. I. 

 figs. 11 & 12). 



On the aboral faces of the sutures, on the contrary, there is a 

 series of depressions resembling pits or elongate holes, and the 

 presence of a few knobs in the midst is evident. 



This arrangement of the adoral aud aboral faces of the sutures 

 resembles, to a certain extent, the do welling of Temnopleurus, de- 

 scribed by one of us in a former communication to the Society, 

 but the knob-and-socket structure is not so perfect *. It is 

 evident that the knobs and elongate processes fit into the sockets 

 and holes of the face of the plate which is opposed to them. 



* Journ. Linn. Soc. vol. xvi. p. 343 (1881). 



