166 



that the anterior segments of the skull tend to undergo fission in the 

 direction of the axis of the chain of segments, and that the cleft 

 separating the maxillary lobe of the embryo from the middle and 

 lateral frontal lobes is morphologically horizontal lying between the 

 hsemal and neural elements of more than one segment ; and in proof 

 of this view a variety of evidence is adduced. 



II. " On the Structure and Growth of the Tooth of Echinus." 

 By S. JAMES A. SALTER, M.B. Lond., F.L.S., F.G.S. 

 Communicated by THOMAS BELL, Esq. Received March 5, 

 1861. 



(Abstract.) 



The author commences his paper by stating that the researches 

 upon which it is based were made more than four years since, and 

 then without the knowledge that the structure had been previously 

 investigated by others. 



An abstract of the literature of the subject (contained in very 

 narrow limits) is then given. 



In 1841 Valentin, in Agassiz's Monograph on the Echinoderms 

 (Anatomic des Echinodermes) , published a description and many 

 good figures of the minute anatomy and growth of the Echinus-tooth. 



Professor Quekett, in his 'Lectures on Histology' (1854), referring 

 to the minute mature anatomy of the organ, states its ultimate struc- 

 ture to resemble bone and dentine of vertebrata. 



Dr. Carpenter, in his work ' On the Microscope,' speaks of the 

 tissue of the tooth as essentially of the same nature as the shell of the 

 Echinidee generally (1856). 



Lastly, Professor W.C. Williamson describes the subject more fully 

 than his predecessors, entering into the question of the development 

 of the tooth both generally and histologically (though apparentlyin 

 ignorance of Valentin's Essay), in a paper on the " Histology of the 

 Dermal Tissues," &c., in the British Journal of Dental Science, 1857. 



The coarse anatomy and relations of the Echinus-tooth are then 

 described, and the question is discussed as to how far the organ 

 resembles and how far it does not resemble the incisor tooth of a 

 Rodent mammal, to which it has constantly been likened. 



