Tol. 54.] MISS J. DONALD OX THE GENUS ACLISINA. 47 



is nearly entire, and is strongly sigmoidal ; it retreats from the 

 suture above, so as to. give almost the appearance of a broad and 

 shallow sinus ; it then comes prominently forward and retreats 

 again anteriorly. This shell accords more nearly with the generic 

 description of Aclisina, De Kon., than either of the others asso- 

 ciated with it, and it therefore appears advisable to regard it as the 

 type of the genus, more especially as some palaeontologists have 

 since referred similai fossils to Aclisina. The description of the 

 form of the outer lip, however, needs emendation, as De Koninck 

 states that it is not prominent, but in this and allied species it 

 certainly is prominent. 



Representatives of the three genera occur in the British Isles, 

 and often in a wonderfully good state of preservation. The Scottish 

 specimens deserve special mention, many of them having the proto- 

 oonch intact, and some also showing the lines of growth distinctly. 

 The examples from Glencart and Law, Dairy, and also from Craw- 

 field, Beith, i^.yrshire, are of a creamy colour, and look more like 

 shells of Tertiary or even more recent age, instead of being so 

 ancient as the Lower Carboniferous Period. Dr. John Young has 

 written a most interesting paper upon the Glencart fossils, published 

 in Proc. Nat. Hist. Soc. Glasgow, 1882, p. 234. He states that 

 these fossils are found in cavities filled with clay in weathered 

 calcareous shale-beds, which were discovered by Mr. John Smith, of 

 Kilwinning. He conjectures that these cavities were originally 

 calcareous nodules which enclosed the organisms lying in the stratum 

 in which the nodules were segregated, aild that the calcareous 

 matter was afterwards dissolved by the passage through the beds 

 of water containing carbonic acid, which left behind only the clay 

 originally mixed with the lime. The fossils remained so free in the 

 day that they could easily be disengaged by washing. 



The shells from Law and Crawfield were obtained from fissures 

 .and partings in limestone and shales, where the rock had become 

 rotten in situ through the percolation of surface-water in recent 

 times. 



It is worthy of remark that many of the Scottish Carboniferous 

 gasteropoda are of much smaller size than their representatives in 

 England and Belgium ; this discrepancy may be owing to the 

 different physical conditions that prevailed in the two areas. 



I am greatly indebted to Prof. Hughes, Mr. James Bennie, Dr. 

 John Young, Dr. Hunter-Selkirk, Mr. James Thomson, Mr. John 

 Smith, Mr. Neilson, Mr. Platnauer, and M. le Chanoine de Dorlodot, 

 for the loan of specimens. I must also record my gratitude to all 

 those in charge of the public collections which I have had occasion 

 to consult, for rendering me every assistance in their power while 

 preparing this paper. I may especially mention Mr. E. T. Newton, 

 Mr. H. A. AUen, Mr. R. B. Newton, Dr. Traquair, Prof. Cole, 

 M. Beclard, Mr. Howse, and Mr. Goodchild. In the conchological 

 department of the British Museum (Nat. Hist.) Mr. E. Smith 

 and Mr. Pace have given me special facilities for studying recent 



