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found in the proximal part of the aperture, e. g. in Hincksiella pulchella (PI. XII, 

 fig. 9 a), Scuticella margaritacea (PI. XX, figs. 3 a— 3 1), Sc. ventricosa, var., Cribri- 

 cella rufa (PI. XII, figs. 7 c, 7 f), the species of the genus Calpidium (PI. XX, fig. 

 11a) etc., a much larger number show more or less distinctlj^ that such a sinus, 

 which I propose to call the ^sternal sinus«, must have been present at an earlier 

 stage but has later on become wholly or partly filled by outgrowths from the 

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

 and Strophipora Harveyi are the only ones that show no traces of such a sternal 

 sinus, while the most indistinct traces are found in the genus Scuticella, 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 the two ribs 

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

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

 more distinct in Sc. urnula (PI. XX, fig. 1 a) and Pterocella alata (PI. XX, fig. 4 a), 

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

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

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

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

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

 very deep, primary sternal sinus is cut off, 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. aurita show a distinct internal hollow. In the genus 

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

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

 tinct internal hollow, which is but rarely seen in the short rudimentarjf ribs, 

 found in the majority of the species of this family. Thus in Scuticella sacculata 

 (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, internal, 

 rounded calcareous lamina, which may I think justifiably be defined as a crypto- 

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

 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 part of this margin, 

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

 to spring from the margin of the sinus. In this way it springs wholly or partly 

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



