81 



aperture itself but either by the aid of hinge-teeth, or where such are missing 

 by the points of suspension of the operculum. Again, the edge of the aperture 

 is divided naturally into two different portions, a distal and a proximal, which 

 meet at an angle on each side. The distal portion then generally forms a larger, 

 more convex curve, the lateral parts of which may be parallel, converging or 

 diverging, while in the proximal, smaller portion we find all possible conditions 

 between a curve and a straight line. A review of a large series of holostome 

 apertures shows us that forms of apertures such as the circular, the elliptic, the 

 semicircular, etc. are mutually connected by such a number of transitions, that 

 it is quite hopeless to base a systematic division only on the form of the primary 

 aperture, hi his well-known Monograph Hi neks* uses the following designations 

 for the form of the aperture in the holostome genera described by him: »semi- 

 circular«c (Chorizopora, Microporella, Porella), »more or less semicircular*; (Phylactelld), 

 » semicircular or suborbicular« (Micropord), » semicircular or semiellipticaU (Retepora), 

 »suborbicular or semicircular^ (Mucronelld), »suborbicular« (Smittid), »suborbicular 

 or subquadrangular« (Umbonelld), »orbicular or ranging from semicircular to semi- 

 ellipticals (Palmicellarid). The designation ^semicircular* is thus used to charac- 

 terize the form of the aperture in eight of the ten genera here mentioned, either 

 alone or in connection with the designations: i>suborbicular«, »semielliptical« and 

 »subquadrangular«, of which the first appears in the diagnosis of four, the second 

 of two genera. We can easily see that the differences in the form of the aperture, 

 which Hincks put down for the genera mentioned, are too vague and indefinite 

 to be of any use in their distinction. 



In contrast to the holostome the schizostome aperture has on its proximal 

 edge a more or less deep sinus. If we take our starting point from a form such 

 as Arthropoma (Schizoporella) Cecili or Scliizoporella spongites, in which the sinus 

 is very narrow, almost slit like, and if we imagine this as gradually widening on 

 both sides, we will have a series of apertures with varying breadth of sinus until 

 at last this disappears, because its sides run into the lateral edges of the aperture. 

 On further extension the sinus becomes wider than the rest of the aperture, as 

 it is in some of the species referred to the genus Lepralia. Still this picture only 

 gives us a fractional part of the variations, which the schizostome aperture in 

 reality presents, because a similar variation takes place partly in the depth or 

 height of the sinus partly in the shape of its proximal rim, which may some- 

 times be straight and sometimes more or less curved. It is clear, that the schizo- 

 stome aperture ofters a far greater possibility for variations than the holostome, 



») 22. 



