286 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



and in PtHometra mulleri. In the latter they first appear at an extraordinarily late 

 developmental stage. 



Increase in the height of the side plates brings them into contact with the 

 sacculi below them, and in their distal portion with the tips of the tentacles which 

 when folded down lie on the outer border of the ambulacra! groove beneath or 

 beyond the outer edge of the covering plate, and almost over the sacculus above 

 them. The downward extensions of the plates pass around the superficial portion 

 of the sacculi, which at first come to lie in gaps formed by the excised lower corners 

 of two adjacent plates, and later in holes formed by two apposed rounded notches 

 just below or even at the middle of the borders of two adjacent plates. In some 

 cases the plates may grow entirely around the sacculi, leaving them protruding 

 through more or less circular holes near the proximal border, as in M onachometra 

 fragUis and in Pcecilometra scalaris; but in the types in which this occurs all 

 stages may always be found between the isolated perforation and the usual two 

 apposed notches. Even when the sacculus reaches the exterior through a perfora- 

 tion near the proximal border of the side plates the adjoining distal edges of the 

 adjacent side plates are notched, since, as a result of the contour of the side plates 

 and of the position of the sacculi, they come in contact with them. 



The uniform outward growth of the side plates is prevented by the tentacle 

 tips, and in each side plate a large semicircular notch is formed in the distal 

 portion of the outer border in which the tentacle tip rests ; this leaves a slender 

 curved horn-like process on the distal outer angle between the tentacle tip and 

 the sacculus. 



On the inner surface of the plates septa appear more or less inclosing the 

 sacculi which lie over the apposed edges of adjacent plates, and to the outer end 

 of the proximal septum the covering plate is attached. 



Whenever sacculi are absent the distal and proximal borders of the side plates 

 promptly reassume their normal form. If a sacculus be absent, but the notches 

 in the ends of the plates present, it indicates that the sacculus has merely dis- 

 charged its contents and that another will soon form in the same place. 



The formation of the supplementary side plates results from the separate 

 calcification of the main proximal portion and the distal border of the lappets. 

 Since the supplementary side plates are formed within the tissue ordinarily inclos- 

 ing the distal border of the side plates with their accompanying ridge, they are 

 in reality nothing but this distal border which has become detached from the 

 main body of the plate ; and, indeed, in certain species the progressive detachment 

 of this portion of the plates may be traced in all its stages. 



The cause of this detachment of the distal border of the plate appears to 

 lie in the compensation, by the swinging outward of the side plate, of the interior 

 crowding caused by the unusually deep downward flexion of the covering plates. 

 This lateral movement of the side plates, pivoting on the proximal border, can not 

 be extended to the distal border of the plate the ridges on the inner side of which 

 pass deeply within the tissues of the pinnule; there thus remains a permanently 

 uncalcified strip of perisome between the mobile main portion of the plate and 

 the fixed distal border. 



