83 



Inuersiuld), Hippothoidae {Haplopoma, Chorizopora), Adeonidae (Adeonellopsis) and 

 Onchoporidae (Onchopora). 



Besides the shape of the aperture we must also consider its teeth-shaped 

 projections, and I have already called attention to the fact, that we can distin- 

 guish between hinge-teeth, supporting teeth, and such protecting teeth as are 

 placed outside the operculum. Teeth-like projections of different kinds are occa- 

 sionally used by different writers, e. g. Smitt, Hincks, Waters, JuUien, as 

 generic or family characters, and JuUien' has for instance founded a family 

 Smittidae merely on the presence of a median tooth (lyrula) and two side-teeth 

 (cardellae). To judge from the name he gives the two side-teeth (cardellae, from 

 cardo a hinge), we would imagine that he regarded them as hinge-teeth, but in 

 Exochella they belong to the peristome, and may even join together with one 

 another or with the median tooth (Ex. longirostris). For the rest the author 

 writes regarding all the three teeth: >»c'est le develloppement du jeune peristome 

 qui contribue a former la lyrula et les cardelles dans la famille des Smittidae«. 

 There is no reason for attaching much systematic importance to these teeth-like 

 projections, and they can at most be used as more or less constant auxiliary 

 characters, particularly in the diagnosis of genei-a. Hinge-teeth seem to appear 

 constantly, but in somewhat varying shape in the genus Smittina and to be 

 wanting in the genus Discopora. In the genus Thalamoporella they are very di- 

 stinct and well developed in a series of species, whilst in other species they are 

 very slightly developed or absent, and they seem to appear very seldom in the 

 genus Holoporella. Supporting teeth seem to appear constantly in the genus 

 Cellularia, but as already mentioned they vary in shape and number. The median 

 tooth, to which the greatest importance has been attached, seems to appear 

 within most families which are rich in species, but in families which are only 

 tolerably rich in species, it never seems to be constant, and it is not even always 

 constant within the species. It is found in the family Escharellidae, in most 

 species ot the genus Escharella, in the family Discoporidae e. g. in Discopora 

 pavonella, D. scabra and D. plicata, in the family Petraliidae e. g. in Petralia casta- 

 nea and P. bisinuata, in the family Holoporellidae e. g. in Holoporella tridenticulata, 

 in the family Reteporidae e. g. in Retepora novae Zelandiae, in the family Adeonidae 

 e. g. in Bracebridgia pyriformis, and in the family Cribrilinidae in a variety of 

 the fossil Membraniporella crepidula Hag. 



As examples of species, in which the median tooth is sometimes present, 

 sometimes absent, we may mention besides the last-mentioned: Porella compressa, 



» 45, p. 52. 



6* 



