OCR Text |
Show 348 MR. R. I. POCOCK ON THE [Apr. 21, probable that the Mecicobothriidae first appeared in North America and penetrated thence into South America in the latter half of the Tertiary epoch. The three families just discussed form an ascending series in specialisation, starting with Hexura, which leads from the Dipluridae, passing through the Brachybothriidae, thence to A typus, and culminating with Calommcita, the most specialised of all. Assuming, moreover, that Acattyma and Brachybothrium are identical, it is noticeable that of the six known genera, three are peculiar to the Western (American) hemisphere, two are common to both the Eastern and Western, and one only peculiar to the Eastern, namely Calommcita. In other words, the most primitive genera are exclusively American, one that occupies an intermediate position (Acattyma) extends only as far as Japan, and the two genera of the most specialised Atypidae extend from east to west throughout the Eastern hemisphere,, and one of them, Atypus, to the extreme east of the Western hemisphere. Since we should expect to find the primitive types of a group persisting in the area of its origin and the specialised forms in districts remote therefrom, the modifications being brought about by the varying conditions of existence incident to wandering, North America may be looked upon as the probable home of this section of the Mygalomorphae. If this be the explanation of their distribution, it presents analogies with that of the Camel ida? amongst the mammalia, which originated in the Sonoran area of America, and thence extended into South America and into Central Asia by way of Alaska and Kamchatka. Family C yr t a u c h e n iid je . The genera of Cyrtaucheniidae are exceedingly numerous, and rival the Dipluridae in extent of distribution ; but, owing to the complex and puzzling nature of their relationships, it is not easy to deduce any satisfactory conclusions from their distribution. They are referable to the following sections, which are arranged as nearly as possible according to their lines of descent, starting with the most primitive, i.e. the Nemesise, which have departed least from the Diplurid type. 1. Nemesiae.-The genus Nemesia itself is abundant in the Mediterranean Region, and is met with also in China; Anemesia occurs in Afghanistan, Nemesiellus in S. India, and Sccilidognathus in Ceylon. Hermcicha, Hermcichastes, Lepthereus, Pionothele, and Spiroctenus inhabit S. Africa ; Genysa, Madagascar; Arbanitis, Australia, Tasmania, and New Zealand ; Cantuaria, New Zealand ; Rhcichias, Carterici, and Hermcicha (the last also in S. Africa) in South and Central America. 2. Cyrtauchenii, differing from Nemesia? in the procurvature of the fovea.- Atmetochilus and Damarchus occur in Burma and Sumatra; Cyrtauchenius and Amblyoccirenum in the Mediterranean, the latter also in California ; Pelmcitorycter in E. and S. |