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tinctly chitinized marginal region, which consequently corresponds to the previously 

 mentioned opercular arch. There is also a distinct contrast between the marginal 

 region and the median region of the vestibular layer in the above-mentioned 

 avicularian mandibles of Flastra abgssicola and Onychocella sp (PI. XXII, fig. 3 d), 

 because the marginal portion, which forms the lateral walls in the mandibular 

 cavity is strongly chitinized and shines through the surface of the mandible as 

 two brown ribs converging towards the apex. In the elongated pointed mandibles 

 of Flustra denticulata, Microporella marginata, Schizoporella longirostris and Scuti- 

 cella plagiostoma the vestibular layer is chitinized over the greater part of the length 

 of the mandible, although at the proximal part of the mandible it changes to a 

 softer part, and a longitudinal section through such a mandible (PI. XIX, fig. 10 b) 

 shows that the inner cavity towards the apex of the mandible dwindles to a very 

 fine canal; this seems to suggest that the narrow solid tip is formed by a fusion 

 of the two layers. It is not always, however, that such translucent lines arise 

 from the vestibular layer, as many mandibles may be provided with two distally 

 converging ridge-shaped thickenings which, like the ridges mentioned under the 

 zooecial operculum, are projections from the inner sui-face of the mandible 

 itself. Such converging ridges are found in the mandibles in most species of 

 Porella, in Discopora, etc. 



Time does not permit us to enter into further details here regarding the 

 muscles of the avicularia, and we may just recall that for the movement of the 

 mandible there are abductors or openers, and adductors or closing muscles. 

 While the first are always double, the latter are sometimes single, sometimes 

 double, and in many cases two separated muscles are attached to the mandible 

 by a single tendon. More rarely we also find parietal muscles (Flastra species, 

 Escharoides coccinea). 



Waters, as is known, has shown that the cavities provided with an elongated 

 triangular opening in the extinct Eleidae, which were formerly taken to be 

 ooecia, must in reality have been avicularia-like formations; they differ however 

 from the cheilostomatous avicularia, in always lacking a membranous suboper- 

 cular area. In a number of these species I have found a calcified mandible. 



Ooecia. 



Before giving a comparative account of the structure of the ooecia, we may 

 summarise what the literature and especially the older contains regarding these 

 formations. The first writer where we have been able to find anything about the 



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