Reviews — A Bihliogra'phy of South African Geology. 553 



extreme difficulty of interpreting the sections, held that the 

 appearances were not inconsistent with a simply biramous type of 

 limb, and that the supposed spirals were due to the cutting through 

 of a series of obliquely placed flattened filaments like those on the 

 exopodites of Triarlhrus. Walcott now gives a new restoration, 

 differing in many respects from the former ones. He believes that 

 the existence of spiral structures is fully confirmed by fresh sections, 

 but only one spiral organ is now assigned to each leg, and it 

 is provided with a fringe of long filaments attached one to each 

 coil of the spiral. 



The most important evidence for the existence of this remarkable 

 structure is given by a section of Cahjmene, of which a photograph 

 is reproduced in plate xc, fig. 1, of this memoir. The fringe of 

 filaments is clearly shown, and the bases of the filaments are con- 

 nected by a zig-zag line which might be the section of a spiral 

 structure, but which seems much more likely to be the result of the 

 plane of section grazing the surface of a segmented stem. The 

 diagrams which Walcott gives to illustrate the structure of the 

 exopodites of Cahjmene and Triarthrus (figs. 17 and 19) are already 

 very much alike, and the differences between them would obviously 

 be hard to appreciate in sections. If the fringing filaments were 

 attached to the successive coils of an open helical spiral, it seems 

 very unlikely that they would remain in one plane, as they are seen 

 to do in the sections. The slightest torsion of the support would 

 cause them to assume a helicoid arrangement. For these reasons 

 it seems desirable to hesitate before concluding that the appendages 

 of Cahjmene and Ceraurus were constructed on a plan differing 

 fundamentally not only from those of Triarthrus and Neolenus, 

 but also from those of every other Arthropod, living or fossil, of 

 which the structure is known. 



Many other points of interest and importance are discussed in 

 this memoir, and the further investigations on the same subject, 

 which Dr. Walcott promises, will be eagerly looked for by all students 

 of Arthropod morphology. 



W. T. C. 



A Bibliography of South African Geology. By A. L. Hall. 



Memoir No. 18, Geol. Survey of the Union of South Africa, 



pp. 371. Pretoria, 1922. Price 10s. 6(Z. 

 "jV/rOST geologists are well acquainted with the admirable 

 •^'-^ bibliography of the same subject by Miss Wilman, published 

 by the Philosophical Society of South Africa in 1905. The present 

 work brings the bibliography up to the end of 1920, incorporating 

 the whole of Miss Wilman's references and much new material 

 supplied by her, as well as the author's own collections. The result 

 is a list of 5,794 items. The term " geological literature " has very 

 wisely been interpreted in a liberal sense to include many more 

 or less casual references to geology in writings mainly dealing 



