76a 



KEPOKT — 1884. 



1 III 



..«,,... 



iinrrow relatively to tlio radiuls, aiicl Lear lonj,' iiinnules on every secdiid or tliiid 

 joint, very much as in the Carhoiiil'erous liariivrinns hvrinli-ufi. 



Thaumatocrinm, on ihe otlior hand, allliou<;ii a true (-onialuln, has calyx, 

 interradiulH rcstiii},' directly ou iiersistent basals, and so separatin;;' the tir^^t vadialg 

 laterally, this Ijeinj; a peculiarity which is especially characteristic of eirtaiii 

 lihodocrinidip. A still more extraordinary feature in the calyx of ThaionufiiiriK,,^ 

 is the presence of an anal appendaf^e of the same nature as the Ho-called small 

 lateral jn'ohoscis in Taxurrinns and (hii/c/wcriiiu.''; while three Lower Silurian 

 genera, JhlvrocrinnSflictcorritnis, and XfiKicriniis seem to have possessed a similar 

 structure. Its function a])peurs to have been that of sn])])orthi{,' the anal iiilir- 

 radius of the disc, thf)ugh it was in no way ]iierced for the reception of the hind 

 fjut. There is no trace of such a structure in any other Neocrii\oid hut T/itinmu- 

 iocrinus, and its ap])earance in such a specialised typo as a recent Comutula i.« 

 therefore not a little remarkable. 



As rei^arda the otiier (Jomatula), Antvihm is found in all climates and at all 

 depths, ranf^ini? from the littoral zone down to l',!KJ() fathoms. It has, h()\v>v,r, 

 only been obtained four times hrlow I,:.'U() fiilhoins, and at these p'eat deptlisis 

 only represented hy extremely minute individuals. The same is the case with I'uc 

 teu-rajed rrommlunriiuis, wliieh seems to be fairly abundant in sliallcjw watcv at 

 certain localities in Kerfiuelen's Land ; while dwarfed and almost eolourli'ss imli- 

 viduals were obtained at 1,()U() and at 1,800 fathoms at other localities iu the 

 Southern Sea. Actinmni'trd, on the other hand, is principally limited to the 

 trcoiical and subtropical seas, and has not been obtained more than seven lime* 

 below 200 fathoms, its lowest ranjre yet known beinp G.'i.'} fathoms. Eudiocrinnn. 

 a curious five-armed Comatnla, not otherwise distinguishable from Ant.eihin,h\)i 

 been dredged {it various deptlis in the -\tlantic and Vacific between the littoral 

 zone and 1,050 fathoms ; while the singular type ^Wcrmtf/.v-, which represents a 

 permanently premature stage of other (JomatuUe, is confined to depths betwifn 

 yOO and 600 fathoms, rouglily speaking. 



Thus, then, while liat/a/crinus; IIi/ocriiii(s, and 7'/inn)n(tf>)cruiui> arc tlif 

 crinoids which are more especially characteristic of the abyssal zone below l:M 

 fathoms, Ji/iizoenmis, I'enfacrinus, and three (.'omatulid genera extend down into 

 it from quite shallow water. All live, however, are much reduced in size at tliese 

 great depths, audit is furtlier noticeable that the only sj)ecies of rvntacrinus\i\\K\>, 

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

 has the gi'^atest range being the simple ten-armed Pentacriimx XarefUDtits, 



3. On. the Orifjin of Fresh-Wafer Faunas. 

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



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

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

 probably by the exclusion of free-swimming larva) 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 larvre. Hence, this method of propagation shoidd never, or 

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

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

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

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

 and Faludina 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 compaved 

 with t' 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- 



