Tat; NAUTILUS. Ill 



The shell was first described as Lucina cJiildrenee by Gray in the 

 Annals of Pliilosophy, for 1825, p. 136. Nearly at the same time 

 he referred to its unequal valves in the Zoological Journal, 1, p. 221. 

 In the autumn of the same year Sowerby figured the interior of a 

 right valve in part xxvii of his "Genera" under the name of 

 Lucina childreni. Only in 1828, in the supplement to Wood's Index 

 Testaceologicus, was the shell called Tellina childreni and figured on 

 supplementary plate 1, figure 1. 



The shell was recently collected at Pernambuco by Senor Alfredo 

 de Carvalho and sent to Dr. Von Ihering, who forwarded a speci- 

 men to the National Museum, thus confirming Gray's original 

 locality. On comparison with specimens from Cape St. Lucas, 

 named by Carpenter, it became evident that we had to do with two 

 very similar but distinct species. The rarity of the shell is doubt- 

 less responsible for the delay in discovering the mistake. 



Tlie Brazilian species will of course keep the name given by 

 Gray. To the Cape St. Lucas form we may give the name of 

 Phacoides i^Miltha) xantusi in honor of its discoverer. 



The differences are only apparent on a close scrutiny. The P. 

 xantusi seems to be a smaller species when adult, more rounded, 

 more equivalved and with a shorter ligament. It has a more or less 

 bifurcate and vermiculate radial sculpture, that of P. childreni 

 being finer, more regular and more distinctly divided into fine con- 

 tinuous radial grooves and a microscopic minor sculpture between 

 them. 



As in many other Lucinacea, diiectly under the beaks there is a 

 small impressed area. In P. xantusi this in the right valve projects 

 so as to fill an excavation in the other valve and is so much impressed 

 as to make the beak appear sharper and more produced and to dis- 

 tinctly arcuate the two cardinal teeth. In F. childreni the area is 

 smaller, less impressed, not markedly extended toward the other 

 valve and the teeth remain straight. Outside this area a narrow 

 lunule, concentrically striated and bounded by an incised line, rises 

 almost vertically with a length of 19 mm. and a height of about 2 

 mm. In the Californian species the lunule is very small and bent 

 vertically downward so that in the closed valves it is excavated and 

 not projecting and has a length of about 6 mm. It is almost wholly 

 confined to the right valve. If my specimens fairly represent the 

 species, the posterior area in the Brazilian shell is proportionately 



