1907.] on the Foraminifera. 495 



disposed about a rectilinear axis, (xrowth is as regular as before, but 

 the plan is changed. The genera Biffemrina and CJavulina afford 

 similar examples. Among the MUioUdie the closely connected series 

 of genera Peneroplis^ Orbiculina, OrhitoUtes furnishes an instructive 

 instance. 



[A lantern slide displaying the form and structure of PeneropUs 

 was here shown.] 



The test of PeneropUs is formed of a series of simple chambers 

 disposed in a spiral at first but tending to straighten out in the later 

 stages. 



[A lantern slide displaying the form of several varieties of Orhi- 

 cidina was here shown.] 



In OrhicuJiiia the plan is similar at first, but the chambers soon 

 become subdivided, and also widen out laterally, either on one side 

 of a straight axis or on both. If on one side only we have the 

 characteristic hooked shape of shell from which the name 0. adunca 

 of this highly varying species is derived. If on both sides, the 

 prolonged ends of the widening chambers soon meet one another, and 

 the shell assumes in its later stages the annular arrangement. In 

 OrhitoUtes, this latter is the characteristic mode of growth, though 

 here too the annular plan is preceded by some traces of spiral and 

 rectilinear arrangements. 



[Two lantern slides showing the plans of growth of OrhitoUtes 

 7narginaUs and duplex were here exhibited.] 



In the more highly developed species of this genus, moreover, the 

 chambers increase in thickness, as growth proceeds, in a direction at 

 right angles to the plane of the flat shell, and we have the biconcave 

 discs of 0. compJanata. 



[Two lantern slides showing the plans of growth of 0. comp)lanata 

 were here exhibited.] 



These instances will suffice to illustrate the remarkable phenomenon 

 of multiform tests of foraminifera, and we may now inquire— What 

 light have we on its significance ? 



The assumption in the early stages of life of a type of structure 

 different from that of the adult is a phenomenon widely met with 

 among the higher groups of animals. We know that the throat of a 

 chick of the third day is perforated by lateral slits —gill clefts, and that 

 the arterial trunks between them are disposed for all the world as 

 though the chick were to grow up not an air-, but a water-breather. 

 What is true of the young bird holds true likewise in the Mammalia, 

 including ourselves. 



To take an instance from the Invertebrata : the appendages of a 

 scorpion correspond limb for limb with those of the king-crab Limidus, 

 except that one pair about the middle of the series is absent in the 

 scorpion. It has been shown by Brauer that at a certain stage of 

 development of the scorpion's Qgg, this pair of appendages is repre- 

 sented by a pair of little buds, and that there is a pair of ganglionic 



