8 PROFESSOR A. M. MARSHALL. 



between palseontological and embryological histories, while in some 

 instances a single lucky specimen will afford us, on a particular point, 

 all the evidence we require. 



Great progress has already been made in this direction, and the 

 results obtained are of the most encouraging description. 



By Alexander Agassiz a detailed comparison was made between the 

 fossil series and the developmental stages of recent forms in the case 

 of the Echinoids, a group peculiarly well adapted for such an 

 investigation. The two records agree remarkably in many respects, 

 more especially in the independent evidence they give as to the origin 

 of the asymmetrical forms from more regular ancestors. The gradually 

 increasing complication in some of the historic series is found to be 

 repeated very closely in the development of their existing represen- 

 tatives ; and with regard to the whole group, Agassiz concludes that, 1 

 ' comparing the embryonic development with the palseontological one, 

 we find a remarkable similarity in both, and in a general way there 

 seems to be a parallelism in the appearance of the fossil genera and 

 the successive stages of the development of the Echini.' 



Neumayr has followed similar lines, and by him, as by other 

 authorities on the group, there seems to be general agreement as to 

 the parallelism between the embryological and palseontological records, 

 not merely for Echini, but for other groups of Echinodermata as well. 



The Tetrabranchiate Cephalopoda are an excellent group in which to 

 study the problem, for though no opportunity has yet occurred for 

 studying the embryology of the only surviving member of the group 

 the pearly nautilus, yet owing to the fact that growth of the shell is 

 effected by addition of shelly matter to the part already present, and to 

 the additions being made in such manner that the older part of the 

 shell persists unaltered, it is possible, from examination of a single 

 shell — and in the case of fossils the shells are the only part of which 

 we have exact kuowledge — to determine all the phases of its growth ; 

 just as in the shell of Orbitolites all the stages of development are 

 manifest on nspection of an adult specimen. 



In such a shell as Nautilus or Ammonites the central chamber is the 

 oldest or first formed one, to which the remaining chambers are added 



1 A. Agassiz, Palceontological and Embryological Development. 'An 

 Address before the American Association for the Advancement of Science.' 



