164 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



4. Sfondylium. — The spondylium is an important 

 feature of the pentamerids. It is always of the duplex 

 type as described by Kozlowski, in which the septum 

 is composed of two closely apposed pieces. 



One tendency among the pentamerids, achieved in 

 only two instances, is the reduction of the septum and 

 the elongation of the spondyhum. In extremely ven- 

 tricose forms the rostral chamber is elongated and 

 narrowed, so much so in some species that the sides 

 are only a couple of millimeters apart. This elongation 

 and narrowing of the spondylium is evidently for 

 greater surficial spreading of the diductor muscles, per- 

 mitting a strong pull without a breaking strain on the 

 sides of the muscle platform. 



4. Supporting plates consist of two subparallel 

 septa extending from the middle of the valve and unit- 

 ing with the inner, convex surfaces of the alar processes. 

 It is clear that the function of these plates is to support 

 and make rigid the rest of the cardinalia. In Camer- 

 ella the supporting plates converge inward and unite 

 with a septum. 



Musculature. — Of great interest among the Camer- 

 eUidae is the position of the dorsal adductor scars in 

 front of the cruralium in Camerella and forward of 

 the parallel plates in Par astro fhinella and Anastrophia. 

 This is very different from the situation of the same 

 muscles in the Pentameridas, in which they are located 



26 



27 



28 



Figs. 26-28. — Longitudinal sections of pentamerids. 26, Pentamerus; 27, Pmtameroides; 28, ConchUium s. s. BP, 

 brachial process; 5fiP, base of same; /^, inner plate; Mj, median septum; O/, outer plate; O//, outer plate and septum; 

 Sf, spondylium. 



Dorsal interior. Sefta. — The dorsal interior of 

 the Camerellida is fundamentally different from that 

 of the Pentameridas and will be treated first. Here 

 the dorsal septa can be divided into four more or less 

 distinct parts: (1) the brachial supports, (2) the alar 

 processes, (3) the fulcral plates, and (4) the support- 

 ing lamellae. 



1. The brachial supports appear to be extensions 

 from the postero-ventral extremity of the alar processes. 

 They are blunt and short, and are defined by ridges 

 along the posterior margin of the alar plates. 



2. The alar processes appear as concave plates, con- 

 vex inward, which define the walls of the notothyrial 

 cavity at the posterior. These processes in their for- 

 ward extension curve outward. The alar extensions 

 may represent the complete supports of the lophophore, 

 taking the place of the elongate processes in other 

 forms. 



3. Fulcral plates are small plates, concave dorsally, 

 which define the sockets much as in the Orthoidea 

 (Plectorthidae and Schizophoriidae). 



within the confines of the subparallel or divergent 

 septa or within the subrostral vault of the crurahum. 

 In the Camerellidas the situation of the adductors is 

 much like that of the adductors in the Syntrophiid.-e 

 and Huenellidas, and it is very hkely that future study 

 will show the origin of the Camerellida; out of these 

 groups. This supposition, however, must await the dis- 

 covery of the pallial marks in the Camerellidje and 

 will be decided by the course of these structures on the 

 inner surface of the dorsal valve. 



As stated above, the muscle-scars of the Pentam- 

 erids are borne within the septal plates of the dorsal 

 valve, and in the spondylium of the opposing one. 

 This enclosure of the dorsal muscles by the septa is 

 held by us to be a markedly characteristic feature 

 of the Pentameridx, differentiating them from the 

 CamereUidae. 



Cardinalia. — The cardinalia of the Pentameridx 

 are the most characteristic feature of the genera and 

 probably of the family as well. These lamellae are 



