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Show 1902.] ON THE SPIDERS OF THE GENUS LATRODECTUS. 247 senting the type of an highly-organized reptile, and nearer to the Dinosaurs than any other living type. As in the genus Testudo, the right auriculo-ventricular valve of the Birds is almost always a clearly-defined and semi-lunar muscular flap, practically guarding only one side of the auriculo-ventricular orifice. In Testudo, as in the bird, the opposite side of the ventricle has a perfectly smooth surface, without even the rudiment of a ridge to correspond to the valve which lies on the left side of the auriculo-ventricular orifice. On the other hand, the left auriculo-ventricular aperture is guarded by a completely, or nearly completely, encircling valve in both tortoise and bird. Hardly any change is required to convert the right auriculo-ventricular valve of the tortoise into that of the bird. If no rudiments, such as those described in the present communication, of a septal half to the right auriculo-ventricular valve had been discovered, it might be reasonable to dwell upon the striking but really superficial likeness which this valve in the vast majority of existing birds shows to the corresponding valve in the tortoise. But it seems now to be clear that the simple valve in the bird's right ventricle is not the persistent and simple valve of the tortoise or some lower reptilian type ; but that it has been derived from the reduction of a more complicated valve such as is possessed by the Crocodiles, and was very possibly possessed by the Dinosaurs. The evidence is therefore so far in favour of assigning to the birds an origin from some highly-developed reptilian type. 2. On the Spiders of the Genus Ijatrodectus, Walckenaer. B y FREDERICK PlCKARD CAMBRIDGE, F.Z.S. [Received February 25, 1902.] (Plates XXVI. & XXVII.1) CONTENTS. I. Introduction, p. 247. II. List of Species and Sub-species recognized in this paper, p. 252. III. Alphabetical List of Species described or figured, p. 256. IV. Explanation of the Plates, p. 261. I. INTRODUCTION. The genus Latrodectus of Walckenaer (Tableau, p. 81, 1805), of which the type is L. A-tredecim-guttatus (Rossi), 1790, comprises those very interesting Spiders which, under various local names, have been notorious in all ages and in all regions of the World where they occur on account of the reputed deadly nature of their bite. As to the evidence, there can be no doubt whatever that the inhabitants of those regions suffer frequently from blood-poisoning 1 For explanation of the Plates, see p. 261, |
