1869.] Dr. Cleland on the Variations of the Human SkulL 35 



mud, in which they are supported, — Hoi tenia by a delicate maze of siliceous 

 fibres, which spread round it in all directions, increasing its surface without 

 materially increasing its weight, — Hyalonema by a more consistent coil of 

 spicules, which penetrates the mud vertically and anchors itself in a firmer 

 layer. 



It appeared to the author and to Dr. Carpenter, who had had their 

 attention specially directed to this point as bearing upon the continuity and 

 identity of some portions of the present calcareous deposits of the Atlantic 

 with the cretaceous formation, that the vitreous sponges are more nearly 

 allied to the Ventriculites of the chalk than to any recent order of Porifera. 

 They are inclined to ascribe the absence of silica in many ventriculites, 

 and the absence of disseminated silica in the chalk generally, to some pro- 

 cess, probably dialytic, subsequent to the deposit of the chalk, by which 

 the silica has been removed and aggregated in amorphous masses, the chalk 

 flints. 



The Vitreous Sponges along with the living Rhizopods and other Protozoa 

 which enter largely into the composition of the upper layer of the chalk- 

 mud, appear to be nourished by the absorption through the external sur- 

 face of their bodies of the assimilable organic matter which exists in 

 appreciable quantity in all sea- water, and which is derived from the life and 

 death of marine animals and plants, and in large quantity, from the water 

 of tropical rivers. One principal function of this vast sheet of the lowest 

 type of animal life, which probably extends over the whole of the warmer 

 regions of the sea, may probably be to diminish the loss of organic matter 

 by gradual decomposition, and to aid in maintaining in the ocean, the 

 "balance of organic nature." 



XVIII. "An Inquiry into the Variations of the Human Skull, parti- 

 cularly in the Antero-posterior Direction." By John Cleland, 

 M.D., Professor of Anatomy and Physiology, Queen's College, 

 Galway. Communicated by Dr. Allen Thomson. Received 

 June 15, 1869. 



(Abstract.) 



1 . A method of notation is suggested by which material sufficient for 

 the formation of a perfectly accurate diagram of a skull may be registered 

 by means of a line or two of figures. This is accomplished by marking the 

 vertical and horizontal distance of a number of points from the postauricular 

 depression. 



2. The longest base-lines, from fronto-nasal suture to back of foramen 

 magnum, are found in savage skulls. This base-line is distinctly longer 

 in males than females ; and the proportion which the arch bears to the 

 base-line is greater in children than in the adult. In the Irish, the base- 

 line is short, and the arch extensive. 



3. The mesial base being considered in three parts, viz. length of 



d 2 



