No. 13. — The Holothurians of the Museum of Comparative Zoology. The Synaptinae. By Hubert Lyman Clark. Although the collection of Synaptinae in the Museum of Compara-tive Zoology is not a large one, it is of much interest not merely be-cause of the undescribed forms which it contains, but because of the light it throws on the range of diversity among specimens of long-known species. Moreover, special interest attaches to some of the material because it was collected and described by Semper, while other species were examined by Selenka and some of this material served as types for forms described in hi& important monograph on holothurians published in 1867. There are also cotypes of species described by others. The classification is in fairly satisfactory condition and the study of the M. C. Z. collection does not suggest any changes. The recognized genera seem to be reasonably natural groups although much more material from the East Indian region is needed before the limits of some of them are determined. Specific limits too among the East Indian forms require much clearer indication than the present material permits. The nomenclature seems to have reached a state of con-siderable stability and questions concerned with it call for no discus-sion aside from the matter of the validity of certain forms. The characters upon which the genera and species are based are both external and internal, macroscopic and microscopic, but the most important are the tentacles and the calcareous deposits in the body-wall and tentacles. Each of these demands a few words. The Tentacles. The number of tentacles is typically 12, but in one species it is con-stantly 11 and in at least one it is 10. Whether any species has normally and regularly 1.3 tentacles is still to be demonstrated, but in Synaptula the number ranges from 12 to 15 and in .S. recta, it seems probable that 13 is the normal number. Euapta, Opheodesoma, and Synapta have typically 15 tentacles, while Polyplectana has 25 as the typical number, though many specimens, particularly small ones, have a much smaller number. As for the form of the tentacles they
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