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. Parrijanus, Hall, S. Macbridii, Calvin) as to 
suggest a derivation from the rnultiplicate 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 fiiint, 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, 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. Parry anus 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 {8. 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 
