1919] Barrows: Occurrence of a Rock-Boring Isopod 305 



the green stems and roots of the mangroves, causing the wood to decay. The 

 falling away of these destroyed branches and roots causes the loss of many 

 oysters attached to them. As I rarely observed the Teredo in the green stems, 

 it seems that these Crustacea are the most pernicious form and undoubtedly 

 they prepare the way for the more rapidly destructive Teredo. ' ' 



OCCURRENCE OF SPHAEROMA PENTODON RICHARDSON 



This species, Sphaeroma pentodon Richardson, was first described 

 from collections of the Harriman Alaska Expedition of 1899 (Rich- 

 ardson, 1904, p. 214, fig. 96). The species has also been reported from 

 "marshy ground" along the shore of San Francisco Bay (Holmes, 

 1904, p. 323) and from mudflats at Sausalito, near San Francisco 

 (Richardson, 1905, p. 287). This isopod is extremely abundant at 

 the locality described on San Pablo Bay and is probably common along 

 the Pacific coast. The species may, therefore, follow a versatile life, 

 boring into rocks when ledges of sufficiently soft or fine texture are 

 accessible and also living in marshes and in mudflats. 



The sandstone into which these isopods bore on the shore of San 

 Pablo Bay may be described as "a fine grained, friable sandstone of 

 open texture and not well cemented.' 7 The results of an acid test 

 for calcium were negative. For an examination of a sample of this 

 sandstone I am indebted to Professor A. C. Lawson, Department of 

 Geology, University of California. 



The tuff is even more friable than the sandstone and is "composed 

 of a whitish or light yellowish pumice, partly in fragments, ranging 

 in size from 1 to 50 millimeters, and partly in fine dust" (Lawson, 

 1914, p. 13). The tuff in the region where the boring isopods are 

 found is largely composed of the finer materials with occasional beds 

 almost of the fineness of fine clay. 



In the tuff the isopods bore with little if any apparent localization 

 on account of varying hardness or fineness of constituent beds of 

 material. In the sandstone, however, certain beds, presumably the 

 softer, have been selected in preference to others probably harder or 

 more firmly cemented, though there is no apparent difference in hard- 

 ness. The result both of the activity of the borers and of the friability 

 of these layers of sandstone is that the sandstone beds in which the 

 isopods bore are more rapidly eroded than the intervening beds. The 

 tuff probably presents rock of such consistency as to be readily 

 attacked by the borers, while the San Pablo sandstone is, on the 

 average, somewhat too hard for the borers. The isopods bore, there- 

 fore, onlv into the softer of the sandstone beds. This indicates a 



