ROSS ON EVOLUTIOX. 417 



only of the adult individual, for the earlier stages of the life of the 

 individual represented accurately in a modified form, the earliest 

 Species ot the Series to which it belonged taking on successively the 

 characteristics of the successive Species of such series until it arrived 

 at maturity ; in the keeled group changing from four rounded to 

 eighteen foliated lobes, and in form from an open coil to a com- 

 pletely covered umbilicus, while in regard to ornament it takes on 

 the characteristics of the Series to which it belonsfs in reo'ular sue- 

 cession during the successive stages of its growth: *'In other 

 words there is an unceasing concentration of the adult characteristics 

 of lower Species in the young of higher Species, and a consequent 

 displacement of other embryonic features which had themselves, 

 also, previously belonged to the adult periods of still low^er forms." 

 While the shell-covered Tetrabranchiates, have long been continually 

 decreasing in numbers, in specific forms, in size and in ornamenta- 

 tion, the naked Dibranchiates, rival in size the largest of the extinct 

 Tetrabranchiates, or the largest existing Fishes or Reptilians. Many 

 existing Dibranchiates (such as the Cuttle fishes and Sc[uids) have 

 an internal skeleton or osselet, either calcareous, horny or mem- 

 branous. The Connularia^ fossil osselets, which occur from the 

 Trenton Epoch (of the Low^er Silurian) to the Liassic Epoch (of 

 the Jurassic Period) inclusive, are still abundant and are repre- 

 sented at present by such huge forms as Megaloteuthis harveyi, 

 the oldest remains of the Dibranchiates ; but since only the osselets 

 are preserved it is plain that if the earliest Cephalopods, like 

 the earliest Fishes, had no indurated internal skeleton, (and we 

 know" that in the Calamaries it is often not calcareous and that 

 the Octop{d(B are destitute of it, and the shell is represented by 

 two small rudimentary stylets encysted in the substance of the 

 mantle), they may have existed abundantly without having left any 

 definite traces. It is for a similar reason, doubtless, that the 

 Ascidians though their structure would seem to indicate that they 

 had a very remote origin, have never been recognized as fossils. 

 Indeed it is probable that up to the time — the Devonian Period — 

 w^hen the highest of the then existing Ichfhyopsidia began to have 

 osseous tissue developed in their internal skeletons, no internal 

 indurated tissue ever existed in the highest or central type of any 



