83 



Inuersiiila), Hippothoidae (Haplopoma, Chorizopora), Adeonidae (Adeonellopsis) and 

 Onchoporidae (Onchopora). 



Besides the shape of the aperture we must also consider its teetli-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 

 Stnittiddc merely on the presence of a median tooth (lyrula) and two side-leeth 

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

 cardo a hinge), we would imagine (hat he regarded them as hinge-teelh, but in 

 Exochelld they belong to the jteristome, and may even join togethei- with one 

 another or with the median tooth (K.v. lonyirostris). For the rest the author 

 writes regarding all the three teeth: »c'es( Ic (levelloppemenl du jeune peristome 

 qui contribue a former la lyrula et les cardelles dans la famillc des Smittiddei. 

 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 genera. 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 Tludamnporellri tliey are very di- 

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

 very slightly develo[)ed or absent, ami Ihey seem to ajjpear very seldom in the 

 genus Holopordla. Supporting teeth seem to appear conslanliy 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 s|)ecies, it never seems to be constant, and it is not even always 

 constant within the species. It is found in the family lisclmrclliddr. in most 

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

 pcwonclUt, I). sc(d>r(i and D. plicaUi, in the family Petraliidae e. g. in Petralin casla- 

 neii and /-•. hisiiuicdd, in the family UolopovvUiddc e. g. in Holoporclla Iridcniiciihda, 

 in the family Relefxtridae e. g. in Retepord noude ZeUmdide, in the family Adeonidae 

 e. g. in Brdcchridijid piirifonnis, and in llie family Crihriliniilde in a variety of 

 the fossil Mciubrdiiijiorelld crcjiiduld Hog. 



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

 sometimes absent, we may mention l)esiilcs the last-menlioned : I'luclla coiniircssd, 



■1 



' 45, p. 52. 



