THE CONCHOLOGIST. 



that the cold on the 2nd P'ebruary, 1888, did 

 not reach the depth of twenty-four feet until 

 June 6th, and that the annual range of 

 temperature, though 44-2^ on the ground, is 

 only 37^ at twenty-four feet. Two distinct 

 examples will suffice to make this clear. 



Example i. — The temperature in 1888, on 

 February 2nd, at the Royal Observatory, at 

 various depths : — 



On the grass (minimum), lO'S". 

 I inch in the ground ... 32 •o". 

 ^ c ^ u The iiunimuiii not reached 



Sleet ,, ... 39 "o. till Feb. 4th. 



6 feet ,, ... 43-8". ,, ,, Feb. 7th. 



12 feet ,, ... 447". ,, „ Apr.iSth. 



24 feet ,, ... 48-1°. ,, „June6th. 



The annual range of temperature for 1888 

 was : — 



4 feet above ground in shade ... ... 69'3°. 



On grass 44 "2". 



I inch in the ground ... ... ... 38 •6°. 



3 feet „ 23-3°. 



6 feet ,, I5"i°. 



12 feet ,, lo'2°. 



24 feet ,, 37*. 



E.xample 2. — Reading of thermometer in 

 i860 on July 18th, at the commencement 

 and in the centre of the total eclipse of the 

 Sun,* showing the amount of cold caused 

 by this eclipse and the length of time that 

 occurred before this cold reached various 

 depths : — 



Black bulb thermometer, in 



vacuo in sunshine ... i30'o', this fell 65 "0° 



.. 104-0 ,, 38-5° 



•• 90-5 „ 36-5° 



Ditto, not in vacuo 



On grass 



Half an inch in the ground 78*5 



1 inch in the ground ... 76 '2 



2 inches ,, ... 74-4 

 4 inches ,, ... 73 o 

 6 inches ,, ... 71-3 



5-2^ 

 1-9° 

 0-4' 

 00 



The coldest point was experienced on the 

 ground during the total phase, but it was, 



These observations were recorded by myself at Fuente del 

 Mar near Santander in North Spain, where I had charge of the 

 meteorological observations for the .Vdmiralty in the " Himalya 

 Eclipse Expedition." 



twenty minutes later, half an inch in the 

 ground ; thirty minutes later, one inch deep ; 

 fifty-five minutes later, at two inches deep ; an 

 hour and a half later, at four inches deep ; and 

 this cold had not reached six inches deep in 

 five hours, showing how slowly cold descends 

 into the ground. Before the eclipse was 

 total, Arion ater and Helix pisana were 

 moving about as if it were evening; the 

 latter was exceedingly common. 



SUIRENEWTOX IIaLL, 



%lk February, 1 89 1 . 



ON THE MOLLUSCAN SHELL AND 

 PERIOSTRACUM. 



By H. E. guiLTF.R. 



THE morphology and physiology of the 

 external covering of the MoUusca has 

 often been discussed, but the entire relation- 

 ship of the shell to the animal, of which it 

 is a portion, or protects, is not so familiar 

 a subject. The words " the entire relation- 

 ship of the shell to the animal " are here 

 used in a very broad sense. Through or 

 by what evolutionary phases has the animal 

 become possessed of a shell, and what have 

 been the determining influences exerted in 

 order to produce in the shell itself the 

 extremely numerous varieties of form and 

 colour ? 



Most conchologists are aware that the 

 general structural features of molluscan shells 

 are that they consist of carbonate of lime, 

 with an organic substance, usually ha\ing 

 a laminated texture, and in addition to this, 

 an external layer, consisting of palisade-like 

 prisms placed side by side. Finally on the 

 outer surface of the shell, a horny cuticle, 

 termed the periostracum, is usually present. 



