. Ordovician fossils of north China. Paleontology; Paleontology. Vol I. Grabau—Ordovician Fossils from North China (I) 43 unmistakable constriction, after which the expanding shell of the siphuncle becomes annulated. This suggests that the apical portion of the siphuncle was not enclosed by camerte. The annulations (shown well in specimen fig. 12) are oblique, forming on the side an angle of about 60'^ with the axis of the siphuncle. They become fainter on the ventral side, meeting in a broadly rounded forward curve or saddle. About five of these annulations occur in the space of 20 mm. In tra
. Ordovician fossils of north China. Paleontology; Paleontology. Vol I. Grabau—Ordovician Fossils from North China (I) 43 unmistakable constriction, after which the expanding shell of the siphuncle becomes annulated. This suggests that the apical portion of the siphuncle was not enclosed by camerte. The annulations (shown well in specimen fig. 12) are oblique, forming on the side an angle of about 60'^ with the axis of the siphuncle. They become fainter on the ventral side, meeting in a broadly rounded forward curve or saddle. About five of these annulations occur in the space of 20 mm. In transverse section, the siphuncle appears slightly broader than high, the ventral surface being somewhat flattened. Interior of siphuncle with endosyphosheaths and crystalline lime-filling between them. Two or possibly more of these older endosheaths are indicated. The endocone formed by the final (last-formed) sheath of the most mature individual seen (fig. 12c), is of subcircular section in the upper part, with the ventral surface shghtly flattened. In the lower part, or at least in the earlier sheaths, this ventral surface becomes strongly flattened and the greater part of the endocone lies dorsad of the center (fig. lie). It is continued posteriory in the dorso-ventrally compressed endosiphuncle. The thickness of the last endosheath (the wall of the endocone) is about equal to that of the wall of the siphuncle. In the specimen shown in fig. 11, the dorso-ventral diameter is 31 mm., the corresponding diameter of the endocone is mm., the space between it and the ventral surface being mm., while that between its dorsal surface and the correspond- ing surface of the siphuncle is 8 mm. This point is about 13 mm. from the apex of the endocone. A series of sections of another specimen shows the following relationship. (Text figs. la-e).. Figs 1 a-e. Suocesive crnss-sections of the siphuncle of Piloceras platyvenU'um (For distances apart, and measure- ments see table p. 44
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectpaleont, bookyear1922
