GENUS PYRGOMA. 359 



condition, of P. millepora; and crenatum. As neither the 

 mouth nor cirri, in these three species, offer any noticeable 

 characters, distinct from those in Balanus or Acasta, my 

 ignorance of these organs in the other six species is not 

 important. In the above three species the labrum is deeply 

 notched, with about three teeth on each side of the notch, 

 except in P. millepora?, in which the number is six. In all, 

 the mandibles have five teeth, the two lower ones being small : 

 the maxillae are not notched : the outer maxillae are bilobed. 

 In the Cirri, the rami in the first pair are very unequal in 

 length, the segments being slightly protuberant in the shorter 

 ramus. On the segments of the posterior cirri there are 

 four pairs of spines in P. Anglicum, and three pairs (of which 

 the second and third are short) in P. milleporce. At the dor- 

 sal basis of the penis there is a small straight projecting point. 

 Affinities. — The species (with the exception of the first 

 two) are much more distinct from each other, and more easily 

 determined than is usual with sessile cirripedes ; it is, how- 

 ever, quite useless to attempt naming the species without 

 disarticulating and cleaning the opercular valves. Although 

 these valves differ so greatly in some of the species from those 

 of Balanus and the allied forms, the genus itself, as a whole, 

 does not differ much, except in the shell not being divided 

 into compartments, and in the basis being cup-formed and 

 not generally permeated by pores, — these latter characters 

 being in common with Acasta. With respect to the absence 

 of separate compartments, it should be remembered, that in 

 the same species of Tetraclita we have individuals with the 

 four compartments distinct and furnished with radii, and 

 other individuals without any trace of a suture externally, — 

 the outer lamina of shell (though not the inner) having be- 

 come completely confluent all round. At the commencement 

 of this description, when giving my reasons for uniting the 

 several proposed genera into one genus, I gave a sketch of 

 the affinities of the species : I have only to add, that the 

 following sub-genus Creusia is closely, perhaps too closely, 

 allied to Pyrgoma. 



Geographical and Geological Distribution. — Most of the 

 species are inhabitants of the hot coral-growing zones, in both 

 the eastern and western hemispheres, but more especially, 



