DESCRIPTION OF PEMBATOXON INSULARE GEN. N., SP. N. FROM PEMBA ISLAND (MOLLUSCA PULMONATA, UROCYCLIDAE) By JACKIE VAN GOETHEM INTRODUCTION FROM previous work on slugs of the family Urocyclidae, it appears that the lack of a stimulatory organ (diverticulum atrii, stylophorus or sarcobelum) in different species cannot be the peremptory argument for uniting them in a single genus. For example, in a recent revision of the group (Van Goethem, in press) it was clearly demonstrated that three species formerly united in a single genus, Atoxon Simroth 1888, could be divided into two groups on the basis of a wide variety of morphological characters. These groups have been recognized as separate genera ; the first includes Atoxon fasciatum Verdcourt 1965, while the second is represented by Atoxon meridionals Forcart 1967 and Atoxon bruggeni Forcart 1967. The specimens described below certainly represent a new species and though it lacks a stimulatory organ, it cannot be included in any described genus (e.g. Atoxon). A new taxon is, therefore, described here. Abbreviations used in text : h. height ; M median tooth of radula ; Ig. length ; w. width. SYSTEMATICS PEMBATOXON gen. n.* TYPE SPECIES : Pembatoxon insulare sp. n. DIAGNOSIS : Tail long and strong, with visceral cavity extending to its posterior end. Mantle posteriorly completely fused to the cephalopodium ; covering an internal shell ; mantle aperture present, forming a small, medio-dorsal slit at the posterior end of the mantle. Shell unguiform, presenting a more or less bilateral symmetry ; nucleus at the posterior end, medially ; protoconch oval. Jaw strongly arcuate, with a median projection on the ventral margin. Radula with median and laterals tricuspid ; marginals lacking the endocone, bicuspid (some main marginals unicuspid, the ectocone being reduced or lacking), very slender, much longer than the laterals ; mesocone of the marginals extending far beyond the posterior margin of the basal tooth plate. Pulmonary cavity little vascularized ; heart transverse ; aorta divided in two branches immediately after leaving the ventricle. Reproductive system : epiphallus with a small bursa calcifera and a long tubular caecum ; hermaphrodite gland situated well behind the albumen gland, in the middle part of the tail ; stimulatory organ absent ; penis long, with * Pembatoxon results from the contraction of Pemba and Atoxon. Bull. Br. Mus. nat. Hist. (Zool.) 28, 5 Issued 17 September, 1975
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