22: 



NATURE 



[ynly i8, 1872 



or less the proper rhythmic movement necessary to the production 

 of a musical note, according as the intensity of the agitation of 

 efifervescence is greater or less. 



The dead sound of a cracked glass is probably owing to a 

 similar cause. For in that case, as soon as the vibrations travel- 

 ling round the glass arrive at the crack, tlie e Iges of whicli are 

 wholly or partially in contact, they are transmitted from edge to 

 edge, and as, owing to the friction of the edges one against th .■ 

 other, their vibrations do not synclironise, a reflex wave is im- 

 pinged upon each, having a less velocity than the original wave. 

 This rellex wave .vill correspond to the vibrations caused Ijy 

 effervescence. If the crack be cleanly cut out, so as to separate 

 the edges by a well-defined interval, the glass will again emit a 

 musical note. In the latter case, the sonorous vibrations, on 

 arriving at the cut portion, return by the way they came, 

 synchronising with those which they meet. 



The dead sound of the glass, when filled with honey or treacle, 

 is probably owhig to the circumstance of these fluids being not 

 sufficiently mobile to vibrate in unison with the glass ; and thus 

 they destroy its musical tone as effectually as if they generated 

 an independent and non-synchronous vibration. 



London, July 4 Allen Beazeley 



The Names Cambrian and Silurian in Geology 

 Will you allow me to express, as an humble worker among 

 the rocks of North Wales, my sense of the high valae of the 

 contributions to your pages recently by Prof. Sterry Hunt on the 

 "History of the names Cambrian and Silurian in Geology?" 

 I have long felt — and have not hesitated to express my feeling— 

 that a great wrong was done to Prof. Sedgwick when the North 

 Wales groups of rocks from the Bala Beds to ihe Lingula Flags 



the order of which he was the fust to unravel in that difficult 



region — were unceremoniously engulphed in Siluria. 



It has also appeared to me one of the greatest anomalies ia 

 English geological classification, that the magniliceiit and well- 

 dehned groups of North Wales should be typified by their attenu- 

 ated and broken easterly outcrops in the Silurian district of 

 South Shropshire. What Llandeilo section of Siluria is there 

 that worthily represents the Arenig and lower Bala rocks imme- 

 diately east or west of the Berwyn Mountains? What Caradoc 

 section of Siluria is there at all worthy of the fine series of the 

 Upper B.ala rocks of Glyn Ceiriog ? Of the unworthiness of the 

 scliists near the Stiper stones to represent the Lingula beds 

 of North Wales, Prof. Hunt justly speaks in his papers. 



lam glad that jastice seem; at last likely to be done to the 

 veteran. Prof. Sedgwick, than whom a more philosophical geolo- 

 list I am persuaded does not exist. Let but a sufficient number 

 of scientific men resolve to use his older and truer, because more 

 natural classification, and the justice will soon be complete. 



D. C. Davies 



ON THE VARIATION OF SPECIES AS RE- 

 LATED TO THEIR GEOGRAPHICAL DIS- 

 TRIBUTION, ILLUSTRA TED BY THE ACHA- 

 TI N ELLIN. E 



IT has long been known that island species are usually 

 different from, but allied to, those of the neighbouring 

 continents. Darwin has also made us familiar with the 

 fact that each of the Galapagos Islands has a fauna, and 

 to some extent a flora, of its own. Other explorers have 

 called attention to the somewhat limited distribution of 

 species in the West Indies and on other islands. I have 

 been informed by Mr. T. Bland, who has given special 

 attention to the terrestrial molluscs of the West Indies, 

 that if Cuba should be divided into two islands by the 

 submerging of the central portion, about half of the 

 species on cither of these islands would be different from 

 those on the other. Some of the most remarkable facts 

 of this kind appear in the distribution of the Achatinellin.L- 

 on the Sandwich Islands. As they have never been fully 

 recorded, I make the following brief statement of the 

 leading facts, gathered from the results of personal explo- 

 ration, and suggest a few inquiries. 



Many types of the Sandwich Island Helicida; have at 

 different times been classed under the generic name of 



.■ichatinella. These widely differing forms have, in the 

 structure of the shell, one point of correspondence that 

 holds them together. The columella has a spiral twist 

 which is more or less apparent in all. In most of the 

 species this character is so strongly developed that the 

 columella seems to be armed with a lamellated tooth re- 

 volving within the shell. This common characteristic, in 

 connection with the fact that they are all confined within 

 the limits of one small geographical area, affords sufficient 

 reason fw regarding even the most divergent of these 

 types as belonging to one group. As the humming birds 

 are peculiar to America, so the Achatinellina; are peculiar 

 to the Sandwich Islands. 



Though the forms thus brought together evidently con- 

 stitute a natural group, it has long been apparent that they 

 should be classed under more than one generic name. 

 Some of these genera are restricted to one or two islands. 

 Genera on Kauai 



Several large turreted species of a peculiar type, found 

 only on the island of Kauai, had been provided for at 

 different times under the names ai Achatina,AchatincUa, 

 and Spiraxis ; but no resting-place was found for them 

 till shelter was provided under the separate name of 

 Carelia, given by H. and A. Adams. Carelia tiirricula, a 

 species which is sometimes three inches in length and 

 about an inch in diameter, may stand as the representa- 

 tive of this genus. Besides the six or eight species of 

 Carelia which have been described, there are many other 

 species of land shells peculiar to this island, the most 

 northern and western of the group. Some of these are 

 Helices; the others belong to Amaslra (H. and A. 

 Adams) and Leptaehatina (Gould), two genera which are 

 also represented on the other islands of the group. None 

 of the species of this island present any of the brilliant 

 colours that are so common in the shells of Oahu. The 

 peculiar forms of some of the species, as of .linastra 

 A'lUiaiensis 3.nd Carelia cuiningiana, as well as the relations 

 of these aberrant types to the types found on the other 

 islands, render them objects of great interest. 



Genera on Oahu 



On the island of Oahu, which lies next to K luai on the 

 south-east, we find a remarkable development of the 

 Helicidre. The ground species belong to the two genera 

 just mentioned,. -/widV/v? and Leptaeliatina. .t. vcntnlus 

 is an example of the former, and L. vitrea of the latter. 

 Over twenty-five species of each have been found on this 

 island. Two arboreal genera — the Bultniclla (I'feiffcr) 

 and Helicterella (Gulick) — are found only on this island. 

 The ellipsoidal form, as in B. rosea, characterises the 

 former ; and the conical form, as in H. apienlata, the latter. 

 0( Bulimella there are about thirty known species; of 

 Ileliiterella thirty-five. The different species of Bnliniella 

 present a great variety of colours, ranging from bright 

 green and rose, through yellow, brown, and ash, to simple 

 black and white. The prevailing colours of the Helie- 

 terella are white, black, and brown, variously arranged in 

 bands and stripes. The arboreal genus Aehatinella 

 (Svvainson) may also be regarded as belonging especially 

 to Oahu, as it is here represented by fifty-four species, 

 and elsewhere by but three, which are found only on the 

 island of Molokai, about fifteen miles to the east. Aelia- 

 tinclla prodiicta, about one inch in length, is one of the 

 largest of the family. The .Inrieiilella (Pfeitfer) is a genus 

 of small arboreal species found on Oahu, and also on the 

 islands to the east. Many of them are unnamed ; but 

 those on Oahu probably number more than ten. Anricii- 

 lella auricula is given by Ffeiffer as the type. Two other 

 arboreal genera — Partulina (PfeifTer) and Laniinella 

 (Pfeifter) — which find their chief development on the 

 islands of Maui, Molokai, and Lanai, are represented 

 on this island by three species each. The types, as given 

 by Pfeiffer, are Partulina virgulata, found on Molokai, 

 and Lamindla gravida, on Oahu, 



