462 University of California Publications in Zoology [Vol. 22 



whole mechanism of the crystalline style is essentially similar to that 

 described by Nelson (1918) for other lamellibranchs. The sac is pyri- 

 form and the interior is covered with the characteristic dense mat of 

 long cilia. Extending from the mouth of the sac to its distal end is a 

 small groove in the wall with a slight ridge on one side of it. The 

 cells along this groove and ridge differ from those of the rest of the 

 sac in that they stain deeply with Delafield's haematoxylin. are nar- 

 rower, bear shorter cilia or none at all, and have their nuclei close to 

 the basement membrane instead of near the center or toward the distal 

 ends. From Nelson's description of other species, we conclude that 

 these cells are the ones which secrete the substance of the crystalline 

 style. This groove terminates distally in the orifice of a small appen- 

 dix to the sac (pi. 22, fig. 1). This appendix (a p.. pi. 23, fig. 8) is 

 termed by Sigerfoos (1908) the tubular part of the sheath of the 

 crystalline style. Its thin walls are made up of non-ciliated cells 

 similar to those in the secretory groove and ridge and are continuous 

 with them, indicating that it contributes to the formation of the style, 

 as Sigerfoos suggests. A large fold of epithelium overhangs the open- 

 ing of the sac into the stomach (pi. 23, fig. 6). 



The crystalline style itself is of the usual clear, gelatinous sub- 

 stance. It is shaped like a heavy club, the smaller, or "handle." end 

 of the club projecting across the lumen of the stomach to the opposite 

 wall where it bears against the structure which Nelson (1918) lenns 

 the gastric shield (g. s., pi. 22, fig. 1; pi. 23, fig. 6). This shield is a 

 thin, transparent, cartilage-like plate secreted by the epithelium of the 

 stomach. It may be dissected out or may be seen in microscopic sec- 

 tions. Its posterior and dorsal edges are marked by a faint ridge on 

 the wall of the stomach. Ventrally it extends to the mouth of the 

 sac of the style, and anteriorly it reaches into the lateral pouch and 

 the dorsal caecum (<•/. c, pi. 23, fig. 6). 



Deshayes (1845-1848) does not mention the crystalline style nor 

 the gastric shield. Quatrefages (1849), however, describes the style 

 as lying in the foot. He believes that the second branch, or blind end. 

 of the "siphon" of the first stomach in Deshayes' description is the 

 sac of the crystalline style. This is probably correct as it corresponds 

 almost exactly in position and arrangement with that organ in the 

 species we are studying. Quatrefages describes also a cartilaginous 

 structure on the wall of the stomach which is probably the "fleche 

 tricuspide" of other French authors, or what we now call the gastric 

 shield. Beuk (1899) figures, but does not mention, a long narrow 



