279 



Before leaving this question of the connection of the two orders, 

 there is one point -which is worthy of attention, bearing as it does on 

 the value of resemblance of groups widely removed from each other. 

 There exists between certain Brachiopoda and certain members of the 

 sub-order of Ostreans in the order above, a peculiar correspondence of 

 form and a great similarity of the structural features of the hinge. So 

 close is this resemblance that it affords a degree of approximation in 

 external features not usually found between the two groups which are 

 really so remote from each other. If that hypothesis be correct, which 

 makes the higher form in a series the offspring of the lower, there 

 would seem nothing more likely than that the Lamellibranchiata .were 

 developed out of Brachiopoda. The latter are the earliest in time, as 

 well as the lowest in the scale of development, while the former begin 

 to appear in quantity only when the latter had obtained a high state 

 of development. The two groups are each figured in the other by 

 forms which seem to a general view to bridge over the gulf which lies 

 between them. But how would it be possible to convert this seeming 

 into an actual transition ? What imaginary series of forms could hav e 

 turned around the longitudinal axis so that it would be in a position 

 at right angles to its former place, changing at the same time all those 

 features which would necessarily be altered in such a transition ? It 

 is evident that such a transition would require a series of forms, each 

 of which must present a negation of that very principle of bilateral 

 symmetry which we have found of so much importance. And must 

 we not, therefore, conclude that the series which united these two 

 orders is a series of thought, which is in itself connected, though 

 manifested by two structures which have no genetic relation ? 



Professor Agassiz agreed with the substance of Mr. Shaler's paper, 

 and thought that some confirmation of this idea of the relations of the 

 longitudinal axes could be derived from the Bryozoa. He had long 

 believed the relation of this group to the Brachiopoda to be more 

 intimate than is generally acknowledged ; he thought that the horse- 

 shoe ring of tentacles of these animals corresponds homologically with 

 the brachial coils in the Brachiopoda. 



He thought bilateral symmetry should be distinguished from late- 

 rality, which relates to the disposition of the organs on the sides of the 

 body without reference to symmetry ; in mollusks this laterality is on 

 right and left sides ; in articulates, the weight of the organs is on the 

 dorsal and the ventral surfaces, for which he would employ the term 

 tergality ; by radiality he would signify the radiated arrangement of 

 the organs in radiates, and by cephality the preponderance of the 

 head and its contained organs in the vertebrates. 



