PYRGOMA MONTJCULARLE. 373 



of the shell not unfrequently are a little recurved upwards. Orifice 

 extremely minute, circular. The outer lamina of shell, which is 

 smooth, does not extend to the circumference, and consequently a 

 rather broad, nearly equal border, which is rough, surrounds the whole 

 shell. I have no doubt that, when the shell was alive, this border was 

 covered by a membrane, which, in drying, has curled up and been lost, 

 in the same manner as the strictly analogous but narrow open seam 

 between the basal edges of the shell and the basis in some cirripedes 

 (as in the last two species of Pyrgoma) is protected. The roughened 

 border can sometimes be plainly seen to be formed of normal (PI. 13, 

 fig. 5 e) longitudinal septa having crenated edges, with shorter septa 

 between the longer ones ; but more often the septa are so irregular, 

 and so much branched (5 d), that the whole resembles a mass of moss. 

 Why the outer lamina of the shell in this one species does not nearly 

 reach the circumference of the walls, I cannot conjecture. The ex- 

 tremely irregular, depressed shape of the shell, with the minute cir- 

 cular orifice, and the singular rough circumferential and often slightly 

 reflexed border, together give to this species so peculiar an aspect, that 

 until close examination I did not believe that it was a cirripede. The 

 extreme irregularity of shape depends in great part upon the irregular 

 growth of the Monticularia, in which it is imbedded. 



Internally (fig. 5 b) the walls are smooth, but they are perforated by 

 many quite irregular, small orifices, which have the appearance of having 

 been formed by some boring animal, but really serve, as I believe, to 

 admit threads of corium into certain irregular pores which penetrate 

 the shell. The sheath descends but a very short distance from the 

 orifice: it is closely attached to the walls, and might easily be over- 

 looked. The basis is deep, of an irregular outline, like that of the shell, 

 and is formed by a very thin shelly layer. The largest specimen which 

 I have seen, measuring from the extreme projecting points, was *4 of 

 an inch in diameter. 



The Scuta and Terga (fig. 5/) are calcified together without any 

 trace of a suture; together they form a bow with the two ends curled 

 rather abruptly inwards ; they are both extremely narrow, but furnished 

 with an occludent ledge, twice or thrice as high as the proper valves 

 themselves. This occludent ledge, which is finely hirsute, begins at 

 about one third of the length of the scutum from the rostral angle, and 

 runs to near the basi-carinal angle of the tergum. The scutum itself 

 is curved, with a slip, along the true occkident margin (best seen at the 

 rostral end), lying in a different plane from the rest of the valve, much 

 in the same way as in the scutum of P. milleporce. The adductor ridge de- 

 scends a very little below the basal margin of the valve, and extends for 

 nearly its entire length : this adductor ridge makes the proper valve even 

 narrower than it at first appears. The Tergum is extremely narrow, form- 

 ing merely a border to the occludent ledge; but it is not short, being about 

 two thirds of the length of the scutum. There is no trace of a spur ; 

 indeed, the valve is rather narrower where the spur should have stood, 

 than it is at the basi-carinal end. The scuta and terga are calcified 

 together by their apices. 



Affinities. — Although this species, as above stated, differs so remark- 



