16 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



rib, also termed the diagonal rib. The upper articular rib terminates 

 in the articular furrow of the scutum; rarely there is a weak rib 

 above it, forming part of the occludent border of the plate. 



The fixed scutum and tergum have structures analogous to the 

 lateral plates of Balanidae. There are median raised triangular areas 

 or parietes, and sunken or diversely sculptured sutural areas like 

 the Balanid radii. These structures are well shown in plate 7, 

 figure 2a. The radius and ala along the scutotergal suture of the 

 fixed plates have been homologized by Darwin with the articular 

 ribs of the movable plates, the parietes being then homologous with 

 the lower articular ribs. 



The fixed scutum may be quite plain inside, or there may be a 

 pit for the insertion of the scutal adductor muscle. The lower edge 

 of this pit is sometimes raised, forming an adductor ridge. When this 

 ridge projects as a semicircular or spatulate process, I have termed 

 it a myopJiore. (See pi. 1, fig. 3.) 



The wall plates are indicated by letters on plate 7, as C, carina; 

 R, rostrum; F. Sc., fixed scutum; F. T., fixed tergurn; M. Sc., 

 movable scutum; M. T., movable tergum. 



V. strotnia sometimes excavates its basal support when this is 

 calcareous, according to Dai-win. I have seen no evidence of such 

 action in any American species. The surface sculpture of shells or 

 coral, whether living or dead, seems perfectly sharp and uninjured 

 under the Verrucas I have investigated. 



Verrucas are modified in shape by the form of the support and 

 sometimes by crowding, but the wall plates do not reproduce the 

 irregularities of the supporting surface, as in Balanus, or, at all 

 events, only to a very small degree. On echinoid spines Verruca 

 almost always sits with the carinorostral axis parallel to the spine. 

 On corals the position is more variable. 



Mouth parts. I have not found much difference between the man- 

 dibles and maxillae of the various Verrucas examined. The lower 

 point of the mandible is always multispinose, and the edge of the 

 maxilla very irregular. In these characters Verruca resembles the 

 Lepadidae. 



Cirri. In all but two of the species examined the anterior rarnus 

 of cirri i and ii is from two-thirds to as long as the posterior, the rami 

 of other cirri being subequal. The cirri show great specialization in 

 V. stramia and V. alba. In V. stramia the rami of cirrus i are nearly 

 equal, but in ii and iii the anterior ramus is extremely short, less 

 than half as long at the posterior. V. alba has the anterior rami 

 of cirri i and ii very short, one-third the length of the posterior, that 

 of cirrus iii being three-fourths the length. In this species the cirri 

 are unusually slender. 



