BLASTOIDEA. 303 



down contiguous sides of any given pair of pseud-ambulacra. 

 Thus each ambulacral furrow is formed by the junction of 

 the arms of two " lancet-plates." 



At the summit of the calyx of the Blastoidea are usually 

 six apertures. One of these is larger than the others, and 

 is placed in the centre of the calyx, at the point towards 

 which the grooves of the pseud- ambulacra converge. This 

 central aperture has generally, and doubtless rightly, been 

 regarded as the mouth, but Mr Billings dissents from this 

 view. The remaining five apertures (fig. 178, c) are placed 

 at the summit of each deltoid plate, between two pseud- 

 ambulacra, and embraced each by the fork of one of the 

 lancet-plates. Four of the apertures are equal in size, and 

 -have generally been regarded as " ovarian ; " but they are 

 looked upon by the above - mentioned eminent authority 

 as being connected with the respiratory system, and it is 

 known that the internal respiratory tubes reach the surface 

 by all the five smaller apertures. The fifth is of larger 

 size than the other four, and is usually regarded as partly 

 ovarian and partly anal ; but it is looked upon by Mr Billings 

 as "oro-anal." 



Like the Cystideans, the Blastoidea are not only extinct, but 

 are exclusively Palaeozoic. The Cystideans, however, attained 

 their maximum at a very early period of Palaeozoic time ; 

 whereas the Pentremites flourished most abundantly in the 

 Carboniferous seas, towards the close of the Palaeozoic period. 

 The Cystideans had nearly died out before the close of the 

 Upper Silurian period ; and it is a noteworthy fact that it 

 is just at that point that the Blastoidea seem to make their 

 first appearance. Very sparingly represented in the Upper 

 Silurian rocks, the Blastoidea increase largely in numbers 

 during the Devonian period ; but they attain their maximum 

 of development in the seas of the earlier portion of the 

 Carboniferous period. With one problematical exception, 

 no member of the order is known to have survived the 

 close of the Carboniferous epoch. 



The number of genera included in the order Blastoidea is 

 very small, though there is a considerable number of species, 

 and individuals are often very abundant in particular strata. 



