172 MORSE, CLASSIFICATION OF MOLLUSCA 



In the compound Ascidians like Botryllus, where we 

 have a community of individuals clustering round a com- 

 mon centre, their dorsal as well as anterior regions are at- 

 tached, or, in other words, the ventral and posterior regions 

 are free only. 



Among the Lamellibranchiata nearly all the lower 

 forms, and many of the higher forms are fixed or stationa- 

 ry ; and whether moored by a byssus, buried immovable in 

 the mud, or imprisoned in cells of their own making, it is 

 the anterior end which is fixed. This obtains, with im- 

 portant exceptions. 



The Monomyarians combine in their structure both high 

 and low characters. In their open mantle, and certain 

 other features, they rank high. In their fixed position, the 

 attachment generally springing from the dorsal region, 

 they rank low. For these reasons, I have placed them in 

 the centre (see Plate, Series II, M) not indicating by 

 this their equal value with the other groups, for I doubt 

 if their separation from the Dimyarians is valid, since the 

 large adductor, composed of two elements, would indicate 

 the presence of both anterior and posterior adductors, com- 

 bined in consequence of the excessive shortness of their 

 antero-posterior diameter. The Monomyarians present sin- 

 gular features of analogy with the Brachiopoda. Thus 

 they are generally inequivalve. The viscera are compact- 

 ed toward the dorsal region, and, when attached, they are 

 generally by a process from the dorsal portion, (e. g. Ano- 

 mia) the lowest feature of attachment. In all these in- 

 stances, particularly with Anomia, the analogy is very 

 striking ; it is analogy only, and nothing more, for in their 

 whole structure, and in the relative proportion of their 

 diameters, they present just the opposite extreme. While 

 we have in Brachiopoda the growth laterally, that is, 

 spreading on the sides and depressed dorsally, and the 

 valves, dorsal and ventral, in the Monomyarians we have 

 the other extreme ; the valves are right and left, and the 

 display is on the side, the growth extending ventrally as it 

 were. So narrow are they that in certain forms, Placuna 

 for example, it is almost impossible to conceive the pres- 

 ence of soft parts between the valves. We compare the 

 relative diameters between the Brachiopods and Monomy- 

 arians, to show how unlike they are in this respect. 



