scuucHEin.] CARDINAL AREAS AND ARTICULATION. 97 



di fieri 11 j;" from that of the Neotremat;i and Protreinata. If the charac- 

 ters above pointed out are of superordinal value, it will be convenieut 

 to refer to these divisions as llomocaulia and IdiocauHa, respectively." 



DEVELOPMENT OF CARDINAL AREAS AND ARTICULATION. 



The earliest suggestion of cardinal areas occurs in Iphidea of the 

 Atremata and in the Acrotretida>- of the N'eotremata. In none of these 

 forms, however, is there a true cardinal area comparable with tliose of 

 the Protremata and Telotremata, since it is not bisected by a delthy- 

 rium, nor are deltidial plates developed. A convex pseudodeltidium 

 is often present, but this feature is not homologous with the deltidium 

 of the higher forms. It is due to holoperipheral growth and interfer- 

 ence by the pedicle. In the dorsal valves of primitive genera in both 

 the Atremata and Neotremata growth is hemiperipheral, but in the ven- 

 tral valve of Iphidea, the most prinn'tive known genus of Atremata, and 

 in the Acrotretida; of the Neotremata, growth is holoperipheral. 



The ontogeny of many species of Protremata shows that this order 

 had its origin in some atrematous paterina-like genus. This must have 

 occurred in pre-Cambrian times, since in the Lower Cambrian there are 

 several species of Billingsella, a highly developed protrematous genus 

 when compared with the theoretical Paterina. Kuior(iina v'Dujiilata 

 Walcott, also of the Lower Cambrian, is a more x>rimitive si)ecies than 

 any Billingsella, and it gives evidence as to the course of evolution from 

 the inarticulate paterina-like ancestor to this rudimentary, articulate, 

 long-hinged genus. K. eingiilata in connection with the Indian genus 

 Schizopholis Waagen shows that the opening between the widely 

 gaping valves of Paterina, which was entirely occupied by the pedicle, 

 was partially closed by a gradual thickening of the lateial walls, and 

 there was slowly developed a primitive, ventral, cardinal area. This 

 area and the articulating processes in JiT. cingulata are very rudimen- 

 tary, and are situated at the lateral extremity of the cardinal area; 

 thus this species still retains a very large open delthyrium, much as 

 in the,theoretical Paterina. In Schizopholis this wide fissure is reduced 

 to a narrow triangular delthyrium by the development of a true car- 

 dinal area, and the articulating processes are now no longer 'at the 

 lateral extremities, as in Kutorgina, but are situated more medially. 

 Naturally, in the older Cambrian, complete articulation did not obtain, 

 as in post-Cambrian times. Some of the oldest protrematous species, 

 such as K. cinf/id((fa, BUUngsella wliiiJicJdi, and possibly others, also 

 retain considerable phosphatic material in their shells, but in later and 

 more highly specialized species the shell is decidedly calcareous. 



Some of the s])ecies of Iphidea have the ventral posterior region 



'O/uo? (homos), tSio? (idios), and KavAd? (kaulos) = stem or pedicle common to both valves and pedicle" 



restricted to one valve, respectively. These characters may be retaiued throughout life or restricted to 

 the iiepionic and noanic stages of growth. 



Bull. 87 7 



