Apeil 27, 1900.] 



SCIENCE. 



655 



recording the variations of the earth's mag- 

 netism, and so we may hope some day to 

 get the material that is needed for a satis- 

 factory study of these phenomena. 



Dr. W. van Bemvielen, who has suc- 

 ceeded Professor van der Stok as Director 

 of the Batavian Magnetic Observatorj', has 

 recently issued a new set of isogonic charts 

 for the epochs 1500, 1550, 1600, 1650 and 

 1700. Magneticiaas owe a great debt of 

 gratitude to this enthusiastic and pains- 

 taking investigator for the exhaustive 

 search he has made for old magnetic data 

 in the various European libraries. Only 

 one who is engaged in similar work can 

 appreciate the araount of love and perse- 

 verance necessary for such work. It is 

 also exceedingly gratifying and commend- 

 able that the author gives in his publica- 

 tion the data upon which the charts are 

 based.* 



L. A. Battee. 

 TJ. S. Coast and Geodetic 



StIEVEY. 



EXOTIC 3I0LLUS0A IN CALIFORNIA. 



The number of foreign molluscan species 

 in California has notably increased in the 

 past few years, and includes both terrestrial 

 and marine forms, detected by various col- 

 lectors in and around San Francisco bay. 



With the single- exception mentioned 

 below, the introduction of these exotic 

 forms has been purely accidental, simple 

 incidents in the usual course of business 

 traffic or commercial interchange. 



First, among the land shells we find the 

 well known snail Helix aspersa, a common 

 European species, largely used for food on 

 the continent and familiar to persons v/ho 

 have patronized the restaurants of Paris. 

 This species was intentionally introduced 



*In connection with above paper there were ex- 

 hibited 30 lantern slides, consisting of portraits of 

 prominent magneticians, views of magnetic observ- 

 atory buildings and instruments, and of photographic 

 traces derived from magnetographs. 



or ' planted,' in California over forty years 

 ago by Mr. A. Delmas, of San Jose, Santa 

 Clara county, who brought the stock from 

 France and turned it out among the vine- 

 yards on the west bank of the Guadalupe, 

 a small river that flows northerly through 

 Santa Clara Valley and empties into the 

 southerly end of San Francisco bay near 

 Alviso. The soil where the snails wer& 

 placed is a rich sandy loam and the place 

 well shaded. When the summer heats 

 reach the maximum, the Helices descend 

 into the ground several feet, hiding in the 

 cracks that form, as the ground dries, and 

 the gopher-holes also furnish cool retreats 

 and protection. The region above named 

 is one of exceeding fertility. It was settled 

 by a few French families. The introduc- 

 tion of H. aspersa by Mr. Delmas was made 

 for edible purposes, or in common parlance 

 ' with an eye to the pot.' Mrs. Bush, of 

 the Normal School in San Jose, informs me 

 that the snails have thriven, and have ex- 

 tended their territory from the starting 

 point on the west bank of the stream to 

 the easterly side, and have multiplied to 

 such an extent, that in some instances they 

 are troublesome in the gardens. Mr. Del- 

 mas, the elder, also planted H. aspersa, in 

 San Francisco and Los Angeles. I have 

 never met with it in my collecting raCables 

 in San Francisco or the outskirts of that 

 city, nor heard of its having been detected 

 by any collector. This particular plant 

 was probably a failure, for a more unfavor- 

 able region than that of San Francisco 

 forty years ago, with its cold sea winds, 

 fog, sand-dunes and shifting sands and 

 sparse ligneous scrubby vegetation it would 

 be hard to find. At the present day the 

 chances for success are altogether better, 

 for the greater area of the city is covered 

 by residences, with plats of grass, garden 

 patches and flower-beds which are fre- 

 quently watered and the general conditions 

 are more promising. It would doubtless 



