408 PROCEEDINGS OF THE ACADEMY OF [Sept., 



NOTES ON THE CIRRIPEDE GENUS MEGALASMA. 

 r.Y HEXRY A. PILSBRY. 



The genus Megalasma was instituted by Dr. P. P. C. Hoek^ for a 

 pentaspidian barnacle dredged by the Challenger in the Philippine 

 Archipelago , which was said to differ from Poecilasma by having the 

 umbo of the scutum situated on the occludent margin above the basal 

 angle, whereas in Pcecilasma the umbo is at the basal angle, as in all 

 other LepadirKv. 



In my work on the cirripedes of the U. S. National Museum,- I had 

 occasion to show that Poecilasma, as understood by Hoek and Gruvel, 

 consists of two series of species, (1) those with the sides of the carina 

 narrow (= Pmcilasma Darwin), and (2) those with the sides notably 

 widened towards the base. Moreover, this second group differs from 

 Megalasma only by having the base of the scutum nearly at a right 

 angle with the occludent border, while in the type of Megalasma the 

 basal margin has rotated through an arc of about 90°, bringing it in 

 line with the lower part of the occludent margin. The position of the 

 umbo is morphologically the same in Megalasma and Poicilasma — that 

 is, at the base of the occludent margin proper, where this joins the 

 peduncular margin. The alleged elevation of the umbo on the occlu- 

 dent margin in Megalasma does not exist. It is merely apparent on 

 account of the continuity of the occludent and peduncular margins, as 

 shown in fig. 2a, where the umbo is at u, the margin of the peduncular 

 orifice at po. In my opinion, therefore, the genus Pcecilasma should be 

 restricted to species like those included by Darwin, while all forms with 

 the sides of the carina wide and the internal plate well developed must be 

 transferred to Megalasma. The mere change in the shape of the 

 scutum at the borders of the peduncular foramen is insufficient for the 

 separation of genera. 



Megalasma, in the limits here assigned, is distinguished from Poeci- 

 lasma by the structure of the carina, which has wide sides near the base, 

 and a well-developed oblique plate or septum within the base, bridging 

 across the cavity of the carina, and terminating above in two projec- 

 tions or teeth. The species of Poecilasma chiefly live upon the cara- 



' Challenger Report, VIII, Cirripedia, p. 50, 1SS3. 

 2 Bulletin of the U. S. Nat. Mtis., No. 60, p. 82, 1907. 



