CALCAREOUS SPONGES. 801 



as it is, between the Lelapiidas on the one hand, and the 

 Minchinellidfe and Murray onidse on the other, as due to 

 convergence. 



As to how many of the vast group of extinct " Pharetronid " 

 sponges should be associated with the Minchinellidse and 

 Murrayonidse, and how many with the Lelapiidae, is a question 

 which we cannot attempt to decide, but we think there is evidence 

 to shew that the great majority belong to the basinucleate group, 

 though this opinion, of course, rests only on skeletal characters. 

 It seems highly probable that in past times tlie Dendya line of 

 descent led to the evolution of the dominant Pharetronid group, 

 while at the present day this group has dwindled away and has 

 been replaced chiefly by the now dominant Grantiidse on the 

 apicinucleate line of descent. 



The great antiquity of the Pharetronid group, considering its 

 high degree of organisation, is highly remarkable. It dates back 

 far into the Palfeozoic Epoch, perhaps even to Silurian times 

 [Ulrich, 1889], and almost certainly to Devonian [Zittel, 1878], 

 yo that it seems that the Calcarea had already reached one 

 of their highest states of evolution at the commencement of 

 the Palseozoic Epoch. Throughout the Secondary Period the 

 Pharetrones were dominant, and very numerous genera and 

 species have been described, Avhereas at the present day they 

 are almost extinct, though possibly a few more forms yet remain 

 to be discovered. 



Of course it is quite possible that the predominance of the 

 Pharetrones over other types of Calcarea in past times is apparent 

 rather than real, owing to the fact that they alone, on account 

 of their coherent skeleton, had much chance of being preserved 

 in a fossil condition. Thus there may have been a kind of 

 " geological selection " of these forms in a fossil condition, but 

 it is indeed noteworthy that the apparently much more primitive 

 groups should predominate over these ancient and highly 

 specialised forms to such an extent as they do at the present day. 



It is possible that a fresh outburst of evolutionary vigour on 

 the part of the more primitive persistent groups may have 

 occurred in comparatively recent times. 



Turning now to the Sycettid line of descent, we must remind 

 the reader, in the first instance, that this appears to have given 

 rise to the vast ixiajority of the recent Calcarea. 



The most primitive genus on this line appears undoubtedly to 

 be Sycetta, with its radially arranged chambers standing entirely 

 separate from one another, with no trace of dermal cortex, and 

 without tufts of oxea at the distal ends of the radial chambers. 

 This genus already possesses a well-developed articulate tubar 

 skeleton, the first joint of which is composed of subgastral 

 sagittal triradiates, which appear never to have been developed 

 along the Dendya line of descent, but which are remarkably 

 constant throughout the whole of the Sycettid line, although 



