W. H. Hobbs — Par agenesis of Allanite, etc. 223 



The marked resemblance between the recent Avicularia 

 (Mygale) and the fossil appears to be grounded upon more than 

 trivial characters. Avicularia and allied forms, constituting 

 the Tetrapneumones, with four air sacs in the abdomen as 

 among the Scorpions, eight ocelli, two pairs of spinnerets, and 

 vertically acting mandibles, form a division of the Araneina of 

 much greater significance than a family, and this fact has been 

 recognized in the separation of the order into the Tetrapneu- 

 mones and Dipneumones. The first division embraces the sub- 

 order Territelarise including the families Theraphosidse and 

 Atypidae, (Tarantulas and large hairy spiders), and the second is 

 divided into six suborders and thirteen families, to which be- 

 long all the true spiders. 



The similarity of organs used for sight, locomotion, and pro- 

 curing food, is of much greater importance than the external 

 segmented or unsegmented nature of the abdomen, which 

 structurally and primarily is truly segmented, as shown by the 

 anatomy, embryology and phyllogenetic history. Taking this 

 view, it is seen that a slight extension of the characters gen- 

 erally ascribed to the Araneina, in the single direction of the 

 segmented nature of the abdomen, will admit Arthrolycosa 

 and place it in the division Tetrapneumones. From present 

 information, there seems to be no marked characters which 

 would exclude it from forming a family in the suborder Terri- 

 telariae. 



Mr. J. H. Emerton previously called the attention of Mr. 

 Harger to the general affinities between Arthrolycosa and My- 

 gale {Avicularia) as noted at the end of the original description. 

 At that time, however, the apparent forcipulate character of 

 what was termed a palpus outweighed all other considerations. 



Tale University Museum, June, 1889. 



Art. XXXI. — On the Paragenesis of Allanite and Epidote 

 as Rock-forming Minerals ; by Wm. H. Hobbs, Ph.D. 



The interesting discovery of Messrs. Cross and Iddings,* of 

 the U. S. Geological Survey, that the mineral allanite or orthite 

 occurs widely distributed as a constituent of many varieties of 

 rocks, has placed this mineral in the list of important accessory 

 rock-constituents, and called the attention of American geolo- 

 gists to its distinguishing characters. In Europe, allanite, or 

 orthite, which is the term commonly used in Germany for the 

 same mineral, had already become recognized as one of the 

 rarer constituents of a few rock species. As early as 1860, K. 



* Cross and Iddings, Wide-spread Occurrence of Allanite as an accessory con- 

 stituent of many Rocks, this Journal, III, xxxii, p. 108, Aug., 1885. 



