THE SESSILE BARNACLES. 15 



admirable exposition of its structure and significance by Mr. T. H. 

 Withei's. Taking such a form as Scillxlepas — 



If we imagine ths almost equal development of the rostrum and carina and the 

 suppression on one side of the lateral valves, the scutum and tergum would be allowed 

 to form that side of the wall, and the opposing scutum and tergum would have to 

 lean over at a greater angle to meet them. We should then have only to suppress 

 the subcarina, the median latus, and the peduncle to get a form such as Proverruca. 

 This was evidently the history of the form, and although Seillselepas may not have 

 been the actual ancestor, it must have been a form somewhat similar. (Withers.) 



Further reduction of the wall by loss of the lateral plates, together 

 with the more efFicient articulation of the remaining wall compart- 

 ments, are all that is required to transform Proverruca into Verruca. 



Genus VERRUCA Schumacher. 



1817. Verruca Schumacher, Essai d'un nouveau systeme des habitations dea 



Vers Testac^s, pp. 35, 91. Monotype, Verruca str6mn=Lepas stropiia Miiller. 

 1817. Clysia Savigny, Leach, Journal de Physique, de Chimie, d'Histoiro natu- 



relle et des Arts, vol. 85, July 1817, p. G9. Monotype, C. striata=Balanus 



striatus Penn. 

 1817. Ochlhosia Ranzani, Opuscoli Scientifici, vol. 1, p. 275; 2, 1818, p. GG. 



Monotype, 0. stroemia. 



1824. Clisia Leach, Encyclopedia Britaiinica Suppl., vol. 3 (ac'cordiug to Darwin). 



1825. Clisia Leach, Zoological Journal, vol. 2, p. 210. 



1827. Clitia Leach; Sowerby, Genera of Shells. Type, Lepus verruca Gmclin. 

 1854. Verruca Darwin, Monograph, p. 49G. 



Verrucidae without lateral plates additional to the four composing 

 the wall. Type, V. stroemia. 



The genus is readily distinguished from all other sessile barnacles 

 by the wall composed of four dissimilar plates, the movable top, 

 like the lid of a chest, composed of two plates hinged to the wall 

 along one side. 



Terminology of plates. — The scutum in Ve7ruca is always smaller 

 than the tergum. The inner face is more or less concave, with raised 

 occludent and tergal borders, an impression or pit for the adductor 

 muscle, and very rarely an adductor ridge. Externally the tergal 

 side or area slopes at an angle with the rest of the valve. The upper 

 border of this slope forms an aj)ical ridge, often not visible in a top 

 view. Below this rib there is invariably an articular furrow. This 

 is followed by the articular ribs, variable in number and position. 

 The last one, terminating at the basotergal angle, is the most con- 

 stant of all.* I have termed it the crescent rib, since it defines the 

 semilunar tergal area of the scutum. 



The tergum is trapezoidal, and usually flat or lightly concave 

 within. The exterior is divided into two areas by the last articular 



1 This ril), terminating at the baso-tergal angle, was not counted as an articular rib by Darwin, but Hoek 

 and otliers, I tliioli properly, regard it as articular. 



