1911.] 



NATURAL SCIENCES OF PHILADELPHIA. 



489 



about six recurved bristle-like hairs surrounding a single much 

 larger and longer bristle, which branches freely, and in living or 

 alcoholic material is a prominent feature even to the unaided eye. 

 Being very brittle, these structures are frequently broken off, but 

 their stumps or pores are always evident and show a very regular 

 arrangement. Apparently homologous with the sutural tufts are 

 two similar ones on each side of the head valve, one on either side of 

 the tail valve, and one in the median line in front and behind. In 

 addition there is a second series of much smaller but equally distinct 

 tufts lying just outside of the first and in more or less regular alterna- 

 tion with them, as shown in fig. 1. The long central bristles have 

 a thickened sheath-like base from which are given off slender, 

 more or less recurved, hair-like processes. Altogether there are 

 22 of these major tufts besides an equal or slightly larger number 

 of the minor (alternating) ones. 



Color (in alcohol) a deep rose-pink, which may be either (1) with- 

 out mottlings, or (2) with a stripe of reddish-brown along each side, 

 or (3) with a broad irregular clouding of pale green, accompanied by 

 some lateral spots of brown or Indian red, or (4) with median spots 

 of bright orange-yellow on some of the valves, and lateral markings 

 of Indian red. 1 The typical form seems to be pink with sutural 

 spots of brown and more or less green suffusion. The girdle shows 

 alternating bands of burnt sienna and pale buff. The latter are 

 sutural in position and there are also small intersutural spots of the 

 same color. The interior is rose, paler toward the edges, but not so 

 vivid as in M. (D.) heathii. 



Length of largest specimen, 9 mm. ; width 5 mm. 



Type locality: Off Monterey, Cal., in about 15 fathoms (H. 

 Heath, 1908). Fifteen specimens examined, as follows: 



No. 



11 



3 



1 



Locality. 



Depth. Collector. 



Date. 



Author's 

 Register. 



Off Monterey, Cal. 15 fms. H. Heath. 

 Off Monterey, Cal. 42 fms. S. S. Berry. 

 Pacific Grove, Cal. Low tide. S. S. Berry. 



1908. 

 June, 1906. 

 June, 1906. 



(8) 



(9) 



(10) 



M. (D.) thamnopora seems nearer to the Mopalia heathii of 

 Pilsbry than to any other form known to me, but is so widely diver- 



1 This specimen has the usual ground color of pink with numerous small white 

 spots and a few larger lateral ones of Indian red, but is remarkable in that the 

 central area of the first and second valves only is a clear bright cadmium-yellow. 



38 



