298 SIDNEY F. HARMKK. 



Thu process is ideutictil with that which has been describud 

 above in P]utliy roides episcopal is. The fact that the 

 wall of the compeusatiou-sac becomes tightly pressed against 

 the lateral wall of the zooecium makes it difficult to see its 

 outline in the old zooccia; and the same applies to many 

 other Cheilostomata. But in a decalcified preparation the 

 characteristic parietal muscles can be seen, even in old 

 zooccia, with the utmost distinctness. 



The operculum articulates with two small condyles which 

 are situated just on the distal side of its j)roximal broader 

 part. The appearance of two linear longitudinal submarginal 

 sclerites (fig. 41) is due to the special development of the 

 vertical flange into two lateral ridges, the distal connecting 

 part of the flange being less developed. The occlusor 

 muscles (ocel.) are inserted into these lateral ridges, and they 

 originate low down from the vertical walls of the zooecium, 

 either from the lateral walls or from the distal wall. The 

 divaricator muscle is the distal group of parietal muscles on 

 each side. I am not certain whether the insertion of these 

 is directly into the base of the operculum or into the floor of 

 the compensation-sac close to the operculum. 



The small, non- porous, suboral region of this species may 

 be taken to indicate the former presence of a suboral avicu- 

 larium^ (as in Umbonula verrucosa). This view is con- 

 firmed by tiie fact that a fe^v of the zooccia actually possess 

 a small avicularium with a, semicircular miindible, which, 

 when closed, lies horizontally, and points away from the 

 operculum. 



L. pal I as i ana is one of the forms which has been specially 

 studied by Calvet (1900), many of whose statements I can 

 confirm, although he has not observed the compensation-sac. 

 In the younger zooecium shown in his pi. xiii, fig. 20 (L. 

 foliacea), it appears to me that he indicates the develop- 

 ment of the compensation-sac as an ectodermic invagination 

 at the pro.xinial end of the operculum. In his {)1. vi, fig. 1, 



' Walcib (l!SSi>, |). Kid) calls allcnliuii to llic coiniiioii occurrence ol I lie 

 suljorul avicularium in Clieilostomes. 



