CONCHODERMA AURITA. 143 



near their bases is sub-triangular. I shall presently make 

 some remarks on their functions and manner of formation. 



The Scuta, as well as the other valves, are imperfectly 

 calcified : shape, variable. They usually consist of two 

 lobes or plates, placed at above a right angle to each 

 other, and rarely (fig. 4 c) almost in a straight line ; the 

 lower lobe is more pointed and narrower than the upper ; 

 the two correspond to the lower and middle lobes in the 

 scuta of C. virgata, the upper one being here absent. 



The Terga are developed in an extremely variable 

 degree ; they are often entirely cast off and absent. In 

 very young specimens, they are of the same length with 

 the carina, but after the carina has ceased to grow, the 

 terga always increase a little, and sometimes to such a 

 degree as to be even thirty or forty times as long as 

 carina. When most developed (fig. 4 a) they are not 

 above one third as long as the scuta, to which they lie 

 at nearly right angles ; they consist of imperfectly calcified 

 plates, square at both ends, slightly broader and thinner 

 at the end towards the carina, where they are a little 

 curled inwards, than at the opposite end ; they are not 

 quite flat in any one plane; internally they are slightly 

 concave; finally, I may add, they nearly resemble in 

 miniature the terga of C. virgata. In full grown specimens, 

 the terga almost invariably drop out and are lost; but 

 even in this case, a long brownish cleft in the membrane 

 of the capitulum, marks their former position. The 

 orifice of the capitulum is usually notched between the 

 terga, or between the clefts left by them ; on each side 

 of the notch there is a slight prominence. In some few 

 cases, however, there is no trace of this notch. Behind 

 the terga or the clefts, the great ear-like appendages, as 

 we have seen, are situated. 



Carina, rudimentary (fig. 4) and often absent; it is 

 pointed-elliptical, and is rarely above the ^th of an inch 

 long. After arriving at this full size, calcareous matter 

 is added to the under surface over a less and less area, so 

 that it becomes internally pointed, and finally, in place of 



