THE SESSILE BARNACLES. 107 



Its wide terga and the nearly level summits of tlie radii give 

 tMs west coast barnacle the appearance of B. tintinnahulum. In 

 these characters it certainly differs widely from the original B. con- 

 cavus, which had a very deeply toothed orifice and narrow terga. 

 No estimate of the relation it bears to the very large barnacle of the 

 Oregon Miocene (coosensis Dall) can be formed until the opercular 

 valves of the latter are found. Darwin's plate 4, figures 4a-4c, from 

 Panama, belongs, I believe, to the Californian race, but I have not 

 seen any Panamic or Peruvian specimens. 



B. concavus pacificus attains a size much greater than B. ampM- 

 trite; the scuta differ by their longitudinal striation, the terga by the 

 longer spur and the well-developed, more or less infolded furrow of 

 the exterior. The rami of the first cirrus are but slightly unequal in 

 length, while in B. amphitrite they are very unequal. Finally, the 

 armature of the third cirri is less developed in concavus. 



Of the variations of B. concavus pacificus in California it need 

 only be said that when crowded the cylindric form is assumed. 

 The details of the mouth parts and cirri are from a specimen of 15 

 mm. diameter from Long Beach, one of a group on Trophon triangu- 

 laris. Specimens from Santa Maria Bay, on the western coast of 

 Lower California, are more solid than the type, deep hellebore red 

 with coral pink radii, having some resemblance to B. tintinnahulum 

 coccopoma. The opercular valves are wanting. 



In the specimens from Albatross Station 2939, near Newport, Cal- 

 ifornia (pL 23, figs. 2-2c), both scutum and tergum are narrower 

 than in the type. The adductor ridge is more united with the articu- 

 lar ridge above, and the articular furrow is deep, bridging over the 

 pit of the depressor muscle, which thus becomes tubular. While the 

 actual structure has not changed much, the appearance is quite 

 unlike the ordinary form. See page 337. 



Darwin has noticed the same tubular structure of the depressor 

 muscle pit in Panamic examples. The tergum in the specimens from 

 station 2939 has a much shorter spur than in the type of pacijicus, 

 but varies in different examples; its external furrow is not quite 

 closed. 



The armature of the cirri varies; the larger individuals from sta- 

 tion 2939 examined having more "teeth" on the third cirrus than 

 the smaller one from Long Beach, and also having a few on the fourth 

 cirrus, where I found none in the smaller specimen. 



The oriental form or forms of B. concavus are unknown to me. 

 As it was not obtained by the Sihoga or the Albatross expeditions^ 

 I presume that it is either rare or local. 



