BRACHIOPODA. 29 



Forms with the median fold and sinus without plications. 



These species, in the degree of plication of the sides and the development 

 of the muscular scars, closely resemble the members of the foregoing group. 

 As a rule, the Ostiolati are stouter shells, shorter on the hinge and more ventri- 

 cose than the Aperturati ; their surface is frequently ornamented with fine 

 granules or interrupted radiating striae, and in some instances this linear 

 sculpture is so pronounced (e. g. S. Parryanus, Hall, S. Macbridii, Calvin) as to 

 suggest a derivation from the multiplicate members of Radiati. The cardinal 

 process is developed as a broad, thin, spreading plate, crossed longitudinally by 

 numerous linear depressions ; a feature which is progressively developed from 

 the earlier members of the group toward the syringothyroids. There appears 

 to be no substantial basis for a subordinate grouping of these species ; they 

 were evidently but slightly susceptible to variations in exterior characters. 

 While the cardinal area is sometimes erect and broad, and at others more or 

 less arched, these differences occur within specific limits. Many of the middle 

 Devonian representatives bear a low median sulcus on the fold, which may be 

 accompanied by a broad, very faint, indistinct plication apparent only near the 

 anterior margin of the sinus. Such are S. Oweni, Hall, S. granulosus, Conrad 

 (^ S. granuliferus, Hall), <S'. audaculus, Conrad (= S. medialis, Hall), S. Marcyi, 

 Hall, S. asper, Hall, S. Macbridii, Calvin, S. eurytines, Owen. 



The differences of exterior are quite subordinate ; the majority of the species, 

 exemplified by S. audaculus, have smooth or lamellose plications, which are 

 often marked by a linear depression on the summit of each ; while others have 

 the plications covered with elongate pustules (S. granulosus, S. Marcyi) or erect 

 granules (S. asper). In S. Parryanus the minute elongate pustules are arranged 

 upon the summits of distinct fine radial striae. 



A character which appears at times in other groups, but which here possesses 

 the highest significance, is the gradual development of the callosity or trans- 

 verse plate in the apex of the delthyrium. Originally, and always in the 

 earlier species (S. perextensus. Meek and Worthen, S. macrothyris, Hall, etc.) an 

 accompaniment of adult or senile growth, it eventually becomes a permanent 

 character existing throughout all the later immature growth-phases of the 



