THE BURROWING BARNACLES (CIRRIPEDIA: ACROTHORACICA) 13 



operculum, there is no trace of an orificial knob or of the aggregated 

 calcified layer. 



Fossil acrothoracicans are separated on the presence or absence of 

 a so-called peduncular slit (see section, Fossils, page 134). It is possible 

 that the presence of an orificial knob or of such an aggregated calcified 

 layer would allow a more extensive peduncular slit, more extensive 

 growth away from the attachment area, and hence more flexibility of 

 movement. Rogerella, with an extensive peduncular slit, also has ridges 

 of aggregated shell material still remaining along the edges of the 

 burrow (see fig. 39). 



The length of the aperture, used by Aurivillius (1892, 1894) as an 

 important diagnostic characteristic, is not readily measured precisely. 

 The opercular plates do not extend for the entire length of the aperture. 

 The ventral end of the aperture is lined internally with the comb coUar 

 and ends ventrally perhaps in an orificial velum, or a few small teeth. 

 The dorsal end of the opercular plates are fused together for a short 

 distance, and hence measurements to the dorsal end of the opercular 

 plate are not strictly equivalent to the true apertural length. Neverthe- 

 less, since the female usually lies on its side when being observed free 

 of the burrow, and the exact dorsal termination of the aperture is not 

 discernible, and since moving the female into a position where this 

 termination would be visible could cause more distortion, the figures 

 in this work for apertural length are for the straight-line distance from 

 the ventral end of the aperture to the dorsal end of the base of the 

 opercular plates. This will be longer than the true length of the aperture 

 as measured within the animal. 



Another striking feature of the mantle exterior is the lateral bars. 

 These are variously developed in different families, reaching their 

 maximum development in the Cryptophialidae. 



The more primitive families have lateral, rod-shaped areas reinforced 

 with scale-like chitinous thickenings of the mantle wall, perhaps with 

 associated hairs (fig. 3a). The cryptophialids have well-developed, 

 rolled, chitinous, rod-like bars, with associated ranks of teeth along 

 one or both sides of the bar, ana terminating posteriorly in an array 

 of teeth often set on a raised area (fig. 28a). 



The major lateral bars are approximately in the middle of each side 

 of the mantle, extending from the aperture about half the distance to 

 the posterior margin. In addition, secondary reinforcing bars often are 

 found dorsally, from the apertural area into the cemented disk area. 



The reinforcing bars appear to give support to the abrasive teeth 

 studding the mantle surface. If the abrasion of the shell by teeth on 

 a mantle surface is effected by muscles which only can contract, it 

 would be difficult to push outward against the shell. No major muscle 

 masses are associated with these lateral bars, and this supports the 



