8 CaIvVin Goodrich 



The observations of skilled zoologists with a great deal of field ex- 

 perience have been w^arnings against giving hastily the emphasis of specific 

 names to the thirteen species herein recognized. Ortmann (6, p. 512) says, 

 "Isolation is, in my opinion, a necessary factor in the differentiation of spe- 

 cies and I do not think that a case ever will be discovered where two 

 closely allied species possess precisely the same distribution." Jordan's 

 law (3, p. 547) lays it down that, "Given any species in any region the 

 nearest related species is not likely to be found in the same region nor in a 

 remote region, but in a neighboring district separated from the first by a 

 barrier of some sort." More briefly Ruthven (9, p. 192) declares that "di- 

 rectly related forms on any line of descent occupy neighboring environ- 

 ments." 



Several of the closely allied species of Gyrotoma do appear to have ex- 

 actly the same distribution to all intents and purposes. Unfortunately I 

 have not all the details of Mr. Smith's collecting experiences. But the im- 

 pression is obtained from his letters and from the notes accompanying the 

 labels that a given reef of a series of shoals is not occupied by a heterogeneous 

 assemblage of mollusks, but by some particular form of the genus, each such 

 colony being cut ofif from other colonies of the genus by deep water, quiet 

 water, rolling gravel, by mud or by rocks that in dry seasons are exposed 

 to the air. Thus within a small area, too small to be especially observed 

 by the collector not upon the lookout for it, there may be a distinct differen- 

 tiation of locality, small barriers with as powerful an influence upon Gyro- 

 tomae as barriers quite plain to the eye — all the effects of isolation. 



The Ope:rcui.a 



The opercula of all Gyrotomae are large, leathery, dark red to dark 

 brown. The growth lines are crowded. The surface in instances is more 

 or less marked with microscopic wrinkles which tend to form irregular 

 lines or bands at right angles to the growth lines. The nucleus is indented. 

 Opercula wear down quickly and become irregular in shape so that in the 

 cases of adults the shape is frequently quite different from that in the 

 young. ]n old specimens the nucleus or area of the inner whorls is often 

 completely eroded away. 



In this study the opercula were first examined in connection with the 

 shells. Later they were more carefully re-examined and compared with 

 one another, species by species. There are recognizable differences, but I 

 cannot feel that they are of great importance. The opercula of pyramidatum, 

 spilhnanii^. lezvisii and hendersoni are quite broad and with the inner whorls 

 well within the operculum. Pagoda has a narrower, darker operculum, the 

 apex more blunt. In pumilum, alahamcnsis and cannlfenmi the opercula 

 are mostly of the pagoda form, but have become slightly broader and no- 

 ticeably larger. The opercula of excismn and laciniatum are practically 

 identical, being smaller than in pyramidatum, darker, thinner and broader 

 than in pagoda. The inner whorls are tightly coiled rather than loosely. 

 The opercula of incisum are thin, ragged, variable in shape. In zvalkeri and 

 amplum the opercula are much the same as those of incisum, but in annplum 



