24 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



organisms among the older representatives of the Anomalocystidse. 

 In the dorsal surface of the theca the margin projects but slightty 

 over the terminal aperture. In some cases it is practically flush with 

 the opening. Laterally the margin is considerably extended, forming 

 an appreciable overhang. This mvagination of the margin for the 

 length of the aperture is quite marked. 



There is a very interesting structure connected with the thecal 

 opening that has apparently not hitherto been observed. This is a 

 sort of trapdoor or flap that depends from the upper margin of the 

 openmg. The composition of this flap may not be determined with 

 certainty. In two specimens where preserved it is somewhat raised, 

 and projects beyond the margin of the theca. It maintains its shape, 

 which is that of the aperture, perfectly. This fact and other evidence 

 relative to the hinging of the structure would make it appear that we 

 have to deal with a solid plate. This may have been formed by the 

 fusion of a number of small plates, however. 



It is evident that the function of this plate was to close the terminal 

 aperture. On the lower surface of the plate and running longitudi- 

 nally across it near the outer margin, is a pronounced ridge. When 

 the plate is dropped this ridge rests on the inner floor of the opening. 

 The portion of the plate anterior to the ridge fits over the lower mar- 

 gin of the theca, and the opening is tightly closed. The invagination 

 of the margin above the opening seems not to be without a reason, 

 for if the margin projected uniformly across the anterior end it would 

 be impossible for this plate to be completely raised. 



The mechanics of the operation of this plate are comparatively 

 sunple. A specimen from which the dorsal surface has largely been 

 removed shows the interior of the distal portion of the theca quite 

 clearly. On either side of the opening is a process which extends to 

 the top of the aperture. On these processes the plate was no doubt 

 hinged. On the floor of the theca are two deep longitudinal pits. 

 These probably served as the pomts of attachment for the muscles 

 that raised and lowered the plate. 



The lateral appendages of Placocystis and Anomahcystis are a 

 peculiar and special modification. By Haeckel (1896, p. 40) they 

 have been restored in the case of Placocystis as true brachioles. 

 Bather (1900) considers that such an appendage ''may have served 

 as an arm, i. e., as the bearer of a tentaculate extension of the water 

 system, and of a ciliated path to the mouth." So far as may be 

 observed, these appendages show no signs of being jointed, and in 

 the case of Placocystis, on the authority of Bather, may reach a length 

 of two-thirds that of tlie theca. In Anomahcystis the appendages 

 seem to have been considerably shorter. The spines, as Bather styles 

 them, are somewhat curiously articulated at the distal angles of the 

 theca. 



