Prof. W. K 



in<r on some 



(fig. 2, r), and may thcrcfovc bo called the l)rachlocoele * or 

 brachial chamber. Its up])er and under surfaces (dorsal and 

 ventral lobes of the pallium or mantle) are highly vascular. 



3. The sides of the splanehnocoele in its posterior half, as 

 just stated, are rather strongly incurved, giving rise to two 

 lateral spaces (fig. 1 ; & fig. 2, m), each bounded at the back or 

 inner side by a post-latero-parietal, but open in front or the 

 side corresponding with the adjacent margin of the shell. Pos- 

 sessing no special feature, I propose to give the name pleuro- 

 coeles t to these spaces, simply from their position as side 

 chambers. The two lobes of the mantle forming their upper 

 and under surfaces are highly vascular. 



The organs contained in the different chambers of most 

 importance in the present paper are those that lie next to, or 

 are inserted in, the valves — namely, the parietal band, shell- 

 muscles, liver, genitalia, and some others ; in addition to 

 which the setal band and pedicle require to be described. 



Parietal hand (/>). — The anterior portion, as already re- 

 marked, passes much further forward in the dorsal than in the 

 ventral valve : it slopes backward from the former to the 

 latter. Elsewhere the band passes directly or vertically be- 

 tween the valves. The incurvation of the post-lateral portions 

 is somewhat the deepest in the dorsal valve : these portions 

 are thicker and more muscular than the others. 



In the dorsal valve there are two curving laminar muscular 

 processes (fig. 2, f), each running somewhat vertically from 

 the inner face of the ante-latero-parietals to the medio- 

 longitudinal line of the shell. Huxley has named their 

 homologues in Terehratala the gastro-parietal bands. 



Liver (c). — This occupies the anterior portion of the splaneh- 

 nocoele. Its sm'face-area is smallest in the pedicle-valve, and 

 centrally situated. In the dorsal valve the corresponding area 

 is more expanded, reaching nearly to the ante-latero-parietals : 

 it is divided rather deeply, and in the transverse direction, by 

 the gastro-parietal bands ; while the resulting anterior division 

 in its middle line is slightly grooved by the posterior portion 

 of a medio-longitudinal shelly plate (fig. 2, s), which belongs 

 to the interior of the valve. 



Genitalia (d). — These, which occupy the remaining portion 

 of the splanehnocoele, are distinguished from the liver by a 

 larger pattern of granulation. . Their largest surface-area is in 

 the ventral valve. 



As made known by previous observers, the mouth is 

 situated in the prolongation of the anterior parietal at a little 



* From brachion, arm. 

 t Fix»m i^leura, side. 



