334 



SMITHSONIAN INSTITUTION 



SYNEROCRINUS Jaekel 



Plates XLII; LXXV, figs. 12, 13 



Synerocrinus Jaekel, Zeitschr. d. deutsch. geol. Gesell., XLIX, for 1897 [1898], p. 47.— Bather, Rep. Brit- 

 ish Assoc, for 1898 [1899], p. 923; Treatise on Zoology (Lankester), pt. 3, 1900, p. 190. — Springer, 

 Amer. Geologist, XXX, 1902, p. 95; Jour. Geology, XIV, 1906, p. 519. — Von Zittel, Grundziige 

 Palaeontologie, 1910, p. 166. — Zittel-Eastman, Textbook Paleontology, 1913, p. 205. 



m®wa 





w 



Fig. 44. Synerocrinus 



Ichthyocrinidae with rays above radials separated in lower part by solid 

 plates. Crown elongate to rotund, expanding from radials up. Infrabasals 

 entirely within the ring of basals. No radianal. Anal x united by suture to 

 posterior basal, and more or less to adjacent brachials, followed by a tube. 

 Interbrachials few; rays usually meeting above them, but not interlocking. 

 Primibrachs two. Arms heterotomous; rays in twenty main divisions with 

 simple ramules inside of dichotom. Column large, enlarging proximally. 



Genotype. Forbesiocrinus incurvus Trautschold. 



Distribution. Upper part of Lower Carboniferous; Russia and Scotland. 



Considered with regard to the characters on which family distinction is based, Synero- 

 crinus stands on the border line between the Taxocrinidae and the Ichthyocrinidae. The anal 

 structure is essentially that of a tube bordered by perisome, but this tube does not exactly 

 originate on the posterior basal ; and therefore the genus may be held within its family with- 

 out stretching the definition to the breaking point. Superficially the anal side of the specimen 

 looks like that of a Taxocrinoid (PI. XLII, fig. 6a), but the instructive dissection in figure 8a 

 shows that the actual structure is essentially different. The posterior basal does not directly 

 support the tube, but is suturally united to another plate which does ; and this plate is also 

 suturally united for part of its height to plates of the adjacent rays at both sides. The fossae 

 on the distal face of the posterior basal and the side of the left radial are clearly shown in 

 figure 8a, and the exact relation of the first anal plate to the otheirs may be seen in the sketch 

 of these parts, figure 7, where the tube is drawn without the perisome, which would doubtless 

 appear in perfect specimens. The inner margins of the rays above the sutures connecting 

 them with the first anal plate are rounded just as in the Taxocrinoids, but the posterior basal 



