760 eeporx— 1884. 



narrow relatively to the radials, and bear long pinnules on every second or third 

 joint, very much as in the Carboniferous Barycrinus herculeus. 



Thaumatocrinus, on the other hand, although a true Comatula, has calyx- 

 interradials resting directly ou persistent basals, and so separating the first radials 

 laterally, this being a peculiarity which is especially characteristic of certain 

 Rhodocrinidse. A still more extraordinary feature in the calyx of Thaumatocrinus 

 is the presence of an anal appendage of the same nature as the so-called small 

 lateral proboscis in Taxocrinus and Onychocrinus ; while three Lower Silurian 

 genera, Heterocrinus, Beteocrinus, and Xenocrinus seem to have possessed a similar 

 structure. Its function appears to have been that of supporting the anal inter- 

 radius of the disc, though it was in no way pierced for the reception of the hind 

 gut. There is no trace of such a structure in any other Neocrinoid but Thauma- 

 tocrinus, and its appearance in such a specialised type as a recent Comatula is 

 therefore not a little remarkable. 



As regards the other Comatulse, Antedon is found in all climates and at all 

 depths, ranging from the littoral zone down to 2,900 fathoms. It has, however, 

 only been obtained four times below 1,200 fathoms, and at these great depths is 

 only represented by extremely minute individuals. The same is the case with the 

 ten-rayed Promachocrinus, which seems to be fairly abundant in shallow water at 

 certain localities in Kerguelen's Land ; while dwarfed and almost colourless indi- 

 viduals were obtained at 1,600 and at 1,800 fathoms at other localities in the 

 Southern Sea. Actinometra, on the other hand, is principally limited to the 

 tropical and subtropical seas, and has not been obtained more than seven times 

 below 200 fathoms, its lowest range yet known being 533 fathoms. Eudiocrinus, 

 a curious five-armed Comatula, not otherwise distinguishable from Antedon, lias 

 been dredged at various depths in the Atlantic and Pacific between the littoral 

 zone and 1,050 fathoms ; while the singular type Atelecrinus, which represents a 

 permanently premature stage of other Coniatulas, is confined to depths between 

 300 and 600 fathoms, roughly speaking. 



Thus, then, while Bathycrinus, Hyoerinus, and Thaumatocrinus are the 

 crinoids which are more especially characteristic of the abyssal zone below 1,200 

 fathoms, Bhizocrinus, Pentacrinus, and three Comatulid genera extend down into- 

 it from quite shallow water. All five, however, are much reduced in size at these 

 great depths, audit is further noticeable that the only species of Pentacrinus which 

 occur below 1,000 fathoms are those with comparatively few arms, the type which 

 has the greatest range being the simple ten-armed Pentacrinus Naresianus. 



3. On the Origin of Fresh-Water Faunas. 

 By Professor W. J. Sollas, F.G.S. 



The poverty of fresh-water faunas is not to be explained by the difference in 

 composition or climate of fresh-water areas as compared with marine ; but more 

 probably by the exclusion of free-swimming larvae as a means of distribution, since 

 these feeble and fragile forms are unable to make headway against the current of 

 a river which is always directed seawards. An attached form, introduced by any 

 means into a river, could not establish itself so long as it propagated exclusively 

 by free-swimming larva?. Hence, this method of propagation should never, or 

 only very exceptionally, occur in the case of fresh-watpr forms. Nor, as a matter 

 of fact, does it. In Hydra, fresh-water sponges, and Polyzoa, the young emerge 

 in a complete state from a horny cyst. In Unionida? the characteristic glochidium 

 stage occurs. In most gastropods the eggs are attached by imbed ment in jelly, 

 and Paludina is viviparous. The suppression of a free-swimming larval stage not 

 only occurs in fresh-water, but in some marine invertebrates. This is connected 

 with the fact that the larval stage is one of immense disadvantage as compared 

 with the adult, and to escape from it development comes to take place in seclu- 

 sion. From this a series of other modifications results, such as accelerated and 

 abbreviated development. 



Of the various causes which have led to the transformation of marine into fresh- 



