The Invertebrate Fauna of the Uitenhage Series. 79 



It is highly probable that the shells classed as Gryphaa do not 

 represent a homogenetic group, but are polyphyletic in origin, in- 

 cluding repeated offshoots from an ostrean stock. While, in the 

 progress from Ostrca to Gryphcea, there is diminished duration of 

 the attached habit, so that the typical Gryphaa is fixed by cementa- 

 tion of the left valve only during the young stage, examples are not 

 wanting in which the highly specialised characters of Gryphcea are 

 seen to become modified by the acquirement once again of more 

 prolonged attachment. As an instance of this, reference may be 

 made to the shells which, at the close of the deposition of the 

 Oxford clay in England, seem to have largely replaced the familiar 

 Grypluza dilatata J. Sow. In these modified forms, the attached 

 valve did not become free until the neanic stage had been com- 

 pleted or the adult stage had been well entered upon, and the shell 

 therefore perforce retained a relatively flatter and more ostreiforni 

 aspect.' 1 ' Exogyra imbricata might be thought, on cursory exami- 

 nation, to bear no slight analogy to these : the area of attachment 

 has very frequently a similar relation, in point of dimensions, to 

 the whole fully-grown valve, and it is only on the cessation of 

 attachment that the individual acquires the manner of growth of a 

 Gryplicsa and develops the arcuate form which led Sharpe to institute 

 comparison with Liassic shells. It must be realised, however, that 

 in this African form we have an illustration of the passage from a 

 more complete to a less persistent duration of attachment, in the 

 life of the individual, for a study of the youthful stage shows beyond 

 doubt the exogyrate ancestry, and this may be clearly seen in many 

 specimens in which the nature of the youthful characters did not 

 become masked by the modifications incidental to fixation. The 

 duration of attachment varied very much in different individuals, 

 and no doubt often depended upon the nature and form of the object 

 to which the young shell adhered. In some cases a relatively large 

 area of attachment may retain the impress of some foreign surface, 

 such as that of the large and coarsely ribbed Cucidlcea kraussi 

 Tate, or a pseudo-quadrate Trigonia, in a manner which largely 

 obscures the true nature of the umbonal region. Other individuals, 

 again, seem to have secured themselves to some less suitable or 

 stable surface, and at an earlier stage to have entered upon the 

 period of freedom. In these, as in some which have been attached 

 to an even surface, the characters of the fixed stages are clearly seen 

 to be those of a true Exogyra, and present the strongest contrast to 



* Compare also figure of Gryphcea alligata from the Corallian of Nattheim 

 Quenstedt (1), p. 752, Tab. 91, fig. 25 (1857). 



