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found in the proximal part of the aperture, e. g. in Hincksielhi pnlchella (PI. XII, 

 fig. 9 a), Sculicellu maryarildced (PI. XX, iigs. 3 a — ;}1), Sc. nentricasa, var., Crihri- 

 celln riifd (PI. XII, figs. 7 e, 7 f), the species of the genus Calj)idiiim (PI. XX, fig. 

 11a) etc., a much larger lunnher show more or less distinclly, llial such a sinus, 

 which I propose to call the ^sternal sinus<, must have heen present al an earlici- 

 stage hut has later on become wholly or partly filled by outgrowths from llie 

 margins of this sinus. It seems in fact that the species of the genus Calciutriii 

 and Slrophipora Hnrveiii are the only ones that show no traces of such a sternal 

 sinus, while the most indislincl traces arc found in the genus Sculicellu, most 

 species of which show a short sutural line in the middle of the proximal margin 

 of the aperture (PI. XX, figs. 4 a, 5 a, 5 b), arisen by a concrescence of two short 

 ribs which have filled the sinus, a very small remnant of the latter being gener- 

 ally seen in the form of a little perforation behind the suture. If Ibe two ribs 

 are somewhat projecting, the original extent of the sternal sinus is clearly seen, 

 as e. g. in Scnlicella Wilsoni (PI. XX, fig. 2 a). The vestige of this sinus is much 

 more distinct in Sc. iinuila (PI. XX, fig. 1 a) and Pterocclla alaUi (PI. XX, fig. 4 a), 

 in whicli it is filled by two larger distal and two or three smaller, almost tu- 

 bercle-like proximal ribs. While the sinus in the Cdlpitliiiiu species (PI. XX, fig. 

 11a) is filled by the proximal part of the compound oi)erculum, so also in Clavi- 

 porclla (PI. XII, figs. 3 a— ;ib, PI. XX, figs. 10 a— 10 b) the proximal part of the 

 operculum takes pari in Idling it, but at the same time the proximal part of the 

 very deep, j)rimary sternal sinus is cut olT, in the shape of an oval or slit-like 

 opening, from the part filled by the opercular tongue by two ribs meeting in a 

 sutural line, which in CI. aiirila show a distinct internal hollow. In the genus 

 Cuslicdla the sternal sinus not only attains its maximum size, but the ribs filling 

 it appear in larger numlicr, separated by transverse fissures and showing a dis- 

 tincl internal hollow, which is but rarely seen in the short rudimentary ribs, 

 found in the majority of the species of this family. Thus in Scnlicella Sdcciilala 

 (PI. XII, fig. 2 a) a distinct internal hollow is found in the two lateral ribs. 



To understand the structure of the ribs or spines, which wholly or partly 

 fill the sternal sinus, we must bear in mind that the above mentioned, inteinal, 

 rounded calcareous lamina, which may I think justifiably be defined as a cry|)to- 

 cyst lamina although it is covered by a gymnocyst, starts from the proxinuil 

 margin of the primary aperture at a time when this has not yet become calci- 

 fied, and as the sternal sinus forms indeed a larger or smaller pail of this margin, 

 this cryptocyst lamina must consequently in a greater or smaller extent be said 

 lo sj)ring from the margin of the sinus. In this way it sj)rings wholly or partly 

 from the sternal sinus in the species of the genus Coslicella, while this sinus 



