W. D. Lang — Jurassic Polyzoa. 315- 



In the case of these pegmatites it is probable that the contraction 

 of the parent rock on consolidation would diminish the pressure 

 upon the still fluid veins. These would, therefore, at last consolidate 

 under conditions of temperature, pressure, and also concentration, 

 very different from those under which the main mass crystallised, 

 and a marked difference in texture might then be expected. 



On the whole, I am inclined to think that we have in these 

 Blekinge pegmatites merely a local modification of Brogger's theory- 

 The contorted pegmatites may indeed be the ' aufpressungen ' of 

 a differentiated magma ; not, it is true, in this case, invading cracks 

 and fissures of a consolidated rock, but streaming into the still fluid 

 portions of a neighbouring molten mass. But while differentiation. 

 has thus played an important part in the process, it must not be 

 overlooked that, if this view should prove correct, the final result 

 was, to a still larger degree, due to the imperfect integration of 

 a streaky magma. 



_ VI. — The Jurassic fokms of the 'geneba' Stomatopora ani> 

 Pboboscina. 



By W. D. Lang, B.A., F.Z.S., of the British Museum (Natural History). 



AFTER many months' work at the Polyzoa in the British Museum, 

 the author has been driven to the conclusion that the relation- 

 ships of the Jurassic forms of the ' genera' Stomatopora and Prohoscina 

 have been misunderstood, and that consequently their present 

 arrangement, as put forward in the British Museum Catalogue, 

 is unsatisfactory. 



A detailed examination of all the material available has resulted 

 in the following conclusions : — 



1. The division of the forms into the genera Stomatopora and 

 Prohoscina is unnatural. 



2. The development of a colony (the zoarium of Polyzoa) is 

 comparable with and follows the same laws as the development 

 of the individual (the zooschim of Polyzoa). 



3. Therefore the diagnosis of a form, whether ' species ' or 

 * circulus,' ' is incomplete, and for practical purposes useless, unless 

 the part of the zoarium with respect to its age is specified. 



4. In the ' genera ' Stomatopora and Prohoscina the method of 

 branching is of paramount importance. 



It is intended to take each of the above conclusions and explain 

 by what observations it has been reached, and to what rearrangement 

 of the specimens it tends. To upset the existing order may seem 

 revolutionary, but if by this means a natural grouping is arrived 

 at, if evolutionary series are found, such as have been demonstrated 

 among Brachiopods and Ammonites, if when a new form occurs 

 it is found to fit into one of these series, then the orderly result will 

 justify the radical alterations. 



1 J. W. Gregory: Brit. Mus. Cat. Jur. Bryozoa, 1896, p. 22 : and Mem. Geol. 

 Surv. Ind., 1900, ser. ix, vol. ii, pt. 2, pp. 17-22. 



