THE SESSILE BARNACLES. 255 



ers and adductor ridge. The adductor ridge is very strongly de- 

 veloped, deeply undercut, rather well removed from the articular 

 ridge, but sometimes the space between them is partly filled up above. 

 The articular ridge is rather broad. There are usually five or six 

 strong, oblique teeth on the lower part of the occludent margin, those 

 above having been removed by erosion. 



The tergum is typically about half as wide as long. The carinal 

 half is dark or dark-tinted, like the scutum, the scutal half and spur 

 white within. The basal margin forms an angle with the spur. The 

 spur is moderately wide, with a very obliquely truncate end. 



The great variation in color led Darwin to define a var. nigres- 

 cens "Outer lamina of shell almost wholly removed; the portion 

 preserved and the exposed parietal tubes very dark purple or inky 

 black." I have been unable to satisfy myself that this form is racially 

 distinct from the light colored and gray forms. The opercular 

 valves are similar in all. 



Form floridana: The only deviation of note observed among the 

 specimens seen is in a large lot from the rocks at Lake Worth Inlet, 

 Florida. In these the young individuals, sometimes up to 20 mm. 

 diameter, preserve the surface, which is finely and evenly ribbed. 

 The largest individuals, 25 mm. in diameter, have lost the outer 

 layer. The scuta are pale, only tinted with violet, and the terga, 

 also pale, are decidedly narrower (pi. 59, figs. 6-66). 



On the Pacific coast there is a form of T. s. stalactifera ranging 

 from Cape St. Lucas, Lower California, to Nicaragua (pi. 59, figs. 

 5-56, Mazatlan). The color, rather coarse pores, and characters of 

 the opercular valves seem to be the same as in Antillean examples. 



There is a good deal of variation in the size of the pores, even 

 between individuals of the same group and having the opercular 

 valves practically identical. Plate 59, figure 4, from San Juan del 

 Sur, Nicaragua, illustrates this variation. In an old stock, such as 

 this, which has been separated from its Antillean relatives without 

 much change since the Oligocene or Miocene, it is natural that there 

 should exist a number of collateral varieties, resulting in hetero- 

 geneous colonies. There are also several incipient geographic races. 



Form confinis: A lot taken by the Albatross at St. Georges Island, 

 in the Gulf of California, (pi. CO, fig. 2), resembles T. s. mitteporosa 

 and the Lake Worth Inlet form floridana by having the terga de- 

 cidedly narrower than in typical stalactifera and the spur projects 

 very shortly below the basiscutal angle. The articular ridge of the 

 scutum is narrower, and the articular furrow makes a shallower bay 

 below it. The eroded exterior is gray or cream. This is probably 

 a local race of the Gulf, which may be called form confiwis (fig. 81). 



