BULLETIN OF THE 



posed of two lobes, which have their anterior edges always disconnected, 

 and which correspond to the valves of the shell ; a disk of membrane, 

 variously modified, with its edges fringed with a series of tubular brachia ; 

 a mouth situated within the posterior edge of this disk ; a stomach with a 

 more or less differentiated and anteriorly recurved intestine ; a circulatory 

 system more or less contained within a series of vessels and an atrial 

 system of sinuses or lacunes; with a unilocular heart and usually one or 

 two pairs of accessory pulsatile vesicles ; with the genitalia usually sus- 

 pended in the vascular sinuses and expelling their products through one or 

 two pairs of oviducts opening externally ; nervous ganglia in a ring sur- 

 rounding the oesophagus ; respiring oxygen by absorption through contact 

 of the sea water with the surface of the tissues of the mantle and brachia; 

 dioecious, and exclusively marine. 



This diagnosis comprises all the characters which, after careful consid- 

 eration, I find to be common to all the members of the class. There are 

 other characters which are more or less characteristic of the more familiar 

 forms of recent brachiopods, but which are not characteristic of the group 

 as a whole. Thus, many of the recent forms are attached by a pedicel, 

 while others in the same family are attached by the substance of the 

 valves, and others of nearly allied groups are without an attachment of 

 any kind. The shells of many brachiopods are perforated by minute tubuli 

 lined by ca^cal prolongations of the outer laminae of the mantle lobes, while 

 others in the same family, and perhaps, in some cases, in the same genus, 

 are without these perforations. The mantle edge of many genera is jiro- 

 vided with a more or less closely set border of seta?, while others in the 

 same family are entirely without seta;, and even the same individual, in 

 the earlier stages of its growth (but after the other organs are nearly 

 complete) may be devoid of them. In some brachiopods the setae are 

 stated to be movable, while in others no muscles exist by which they 

 might be moved. In some the blood is colored and in others colorless. 

 The chemical composition of the shell differs in different genera, though 

 in the gi-eat majority it is principally composed of carbonate of lime. The 

 embryonic forms differ widely among themselves, some being segmented 

 and possessing eye-spots like the fry of Pneumodermon and Dentalium, 

 while others are unsegmented. In one genus the pedicel is developed out 

 of the middle of the dorsal area of the embryo, so that the valves both 

 bear a dorsal relation to the animal, while in others it would appear to 

 extend from one extremity of the embryo, when the valves would bear a 

 dorsal and ventral relation respectively. 



It is evident that characters such as these, which are few of family, and 

 none of ordinal value, can have no important bearing upon the classifica- 

 tion of the group and its systematic position as a whole. 



