Bulletins of American paleontology . 454 Bulletin 254 height in some species is the same in juveniles and adults, but inone variant of A. argiis (Gmelin) and most forms of A. tumidum(Dunker) it is higher in the early stages but lower as the adultvolutions become more tumid. Historically, species have been de-fined on the basis of geographical locality; this is valid in the caseof Argohuccmum, which seems to have had sufficient time to dis-perse and develop isolated populations that are morphologicallydistinct. The degree of overlap in shell characters in species of Fusi-triton is greater than
Bulletins of American paleontology . 454 Bulletin 254 height in some species is the same in juveniles and adults, but inone variant of A. argiis (Gmelin) and most forms of A. tumidum(Dunker) it is higher in the early stages but lower as the adultvolutions become more tumid. Historically, species have been de-fined on the basis of geographical locality; this is valid in the caseof Argohuccmum, which seems to have had sufficient time to dis-perse and develop isolated populations that are morphologicallydistinct. The degree of overlap in shell characters in species of Fusi-triton is greater than in Argohuccinum. Similarity of geographicallyseparated forms and the absence of a fossil record in the SouthernHemisphere suggest a more recent dispersal, species being in theprocess of evolution, perhaps, rather than completely separated asin Argobuccinum. Relationships between widespread forms of Fusi-triton are not indicated accurately by classification schemes basedentirely on geographical areas or subregions. Material collected. Text-figure 2. — A comparison of male copulatory organs in Prienc rude,Argobuccinum ranelliformc, and Fusitriton canccllatus. (a). Prienc rude andoperculum () ; (b.) Argobuccinum rauelliforme () and operculum () ;(c.) Fusitriton canccllatus () and operculum (). Penis partly extended in(b), retracted within the mantle cavity in (a) and (c). a, anus; ct,cephalic tentacle; e, eye; f, foot; g, gonad; h, head; m, mantle; o, operculum;osg, open seminal groove; p, penis; s, siphon. Cymatiid Gastropods: Smith 455 from mid-oceanic stations in the Southern Hemisphere is inter-mediate between specimens taken nearer the continents, and fur-ther sampling may show a completely gradational series of Sub-antarctic forms. Axial and spiral, commonly bifurcated, ribs are present on allspecies of Fusitriton. The coarseness and number of elements perwhorl and the nodosity where axials and spirals cross are specific-ally diagnostic. Spiral microsculptur
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