488 



PROCEEDINGS OF THE ACADEMY OF 



[Aug., 



distinctly as in M. (D.) heathii, and due largely to the numerous 

 sense organs which under high power appear with great clearness. 



Anterior valve everywhere very finely granulose, otherwise without 

 distinct sculpture. Central area of posterior valve reduced, but 

 similar in ornamentation to those of the median valves; mucro 

 anterior to the middle of the tegmentum; posterior slope steep, 

 slightly concave; posterior margin of the tegmentum in general 

 semicircular, but rather abruptly (though not extensively) squarish 

 truncate or emarginate at the middle. 



c^ 



Mopalia thamnopora.- 



-1. Outline of entire animal. 2. Girdle spines. 3. 

 Outline of interior of tail valve. 



Anterior valve with 7 slits; intermediate valves with 1,1 slits; 

 tail valve "ischnoid," with a regular crescentic insertion plate cut 

 by 6 slits. Sinus broad, rather shallow; in last valve narrower and 

 minutely crenulate. No median sinus behind, and no indications 

 of an approach to this condition other than the above-noted squaring 

 of the tegmentum. 



Girdle narrow, apparently nude even under a hand lens, but shown 

 by high power to be well clothed above with numerous exceedingly 

 minute, not very crowded, ovoid spicules developed into stout 

 elongate spines at the margin. The spinelets of the lower surface 

 are longer, flatter and more pointed than those of the upper, being 

 somewhat intermediate in character between these and the marginal 

 ones. 



Opposite each suture is a pore from which springs a group of 



