44 INTRODUCTION. 



with the exception of Dentalium, are irregularly twisted, and 

 attached to each other, or to extraneous substances. The first 

 family Dorsalia, contains the genus Siliquaria (plates, fig. 1), 

 known from the Serpulae, by the slit which passes through the 

 whole length of the shell on the upper surface of the tube. 

 The second family, Maldania, has the genus Dentalium 

 (plates, fig. 2), a species of which are commonly known by the 

 name of " tooth shells;" these are regularly formed, curved 

 conical tubes, open at both extremities. The third family, 

 Serpulacea, includes the genera Serpula, Spirorbis, Galeolaria, 

 Vermilia, Spiroglyphus, and Magilus. The only shell that 

 a learner would be likely to place among these incorrectly, 

 according to the system, is the Vermetus (plates, fig. 345), 

 which being regularly spiral at the apicial extremity, has been 

 placed among the Mollusca ; to which situation the whole of 

 the shells under consideration have a better title than is gene- 

 rally supposed. It should be mentioned that the Serpulacea 

 are provided with opercula. 



Class Cirripedes. 



This class constitutes the tenth of invertebrated animals, 

 and receives its name from the jointed and ciliated branchia 

 which protrude between the opercular valves. They are Mul- 

 tivalve shells, and were all included in the single genus Lepas 

 in the system of Linneeus, and are commonly known by the 

 name " Barnacles." Lamarck has, however, divided them 

 into two distinct orders. First, the Sessile Cirripedes, or those 

 which being composed of several valves, joined to each other, 

 side by side in a circle, are attached to each other, or to sub- 

 marine bodies by the basal portion of their own substance, 

 and form a hollow, irregular cone, with the aperture above 

 closed by an operculum consisting of two or more valves. 

 Secondly, the Pedunculated Cirripedes, which are composed of 



