GENERA OF THE SUBORDER ORTHOIDEA 



83 



P. eminfns 

 P. intermeJius 



P. UtU4 



P. obtusus 



P. ovatus 



P. faraUduj 



P. cf. flarut 



P. flanissimus 



P. quinqueraJialus 



P. tenuis 



Prod uctort his kayseri Kozlowski 1927 (South America) 



? Orthis incurvata Lamansky 1905 



Discussion. — Productorthis is the most unique and 

 distinctive of all the Orthacea, being remarkable for its 

 convergence in external form toward Productus, but 

 always lacking the spines of the latter. The productoid 

 characteristics are developed to such a marked degree 

 that Pander described many species of this group under 

 the generic term Productus. However, Von Buch 

 and De Verneuil showed the true orthoid characters 

 of these shells and placed them in the genus Orthis, 

 where they remained until recently, when Kozlowski 

 perceived their uniqueness and separated them from 

 the other orthids under the present designation. 



According to Kozlowski, the most important feature 

 of Productorthis is the loss of the interareas and the 

 delthyrium, both of which are diagnostic characters of 

 the Orthacea. In this connection it is interesting to 

 note that another group of orthids has also lost the 

 interareas, namely, Rhifidomella dubia, which we are 

 referring to our new genus Perditocardinia. But here 

 the suppression of the interarea has taken place by 

 lateral compression, producing a rostrate shell when 

 the reduction is complete. In Productorthis, however, 

 the hinge-line retains its width and the reduction of 

 the interareas takes place by the shortening of their 

 length. 



The reduction of the interareas and the marked 

 ventricosity of the shell in Productorthis have produced 

 important changes in the ventral interior, best seen in 

 deviations of the musculature and cardinalia from the 

 usual type in Orthis. In the ventral valve of the 

 former, only two sets of muscle-scars are visible, a 

 wide central track which Kozlowski believes represents 

 the adductor impressions and a small scar at the base 

 of each dental plate which he considers to be the diduc- 

 tor scars. This disproportionate size of the two sets 

 of muscles is contrary to any arrangement observed in 

 other orthids in the collections studied. It is the 

 writers' view that the wide central track really repre- 

 sents the adductors and the diductors which can not 

 be differentiated into their component parts. Such a 

 condition is seen also in the genus Platysirofhia. The 

 small scars at the base of the dental plates of Pro- 

 ductorthis are interpreted by us as the adjustor scars, 

 since such muscles are commonly seen in other genera 

 of the Orthacea and are always situated at the base or 

 partially on the sides of the dental plates. 



Kozlowski believes that the reduction of the inter- 

 areas and the delthyrium has led to the disappearance 

 of the pedicle as a functional organ. He says:*' 



S.1 suppression chcz Ics formes qui nous occupcnt est 

 d'autrant plus importantc qu'cllc entralne la fcrmcturc du 

 delthyrium ct p.ir consequent la disparition du pcdonculc, 

 du moins comme un organc fonctionncl.** 



According to our observations, however, a pedicle cal- 

 list is present in the apex of the ventral valve, and it 

 clearly indicates the presence of a pedicle although it 

 does not show whether it was functional or not. The 

 resorption of the beak in many of the specimens studied 

 produces a well marked apical foramen, and clearly 

 indicates that the pedicle, by this abrasion, was func- 

 tional during the life of the individual. 



Disarticulated valves might lead the casual observer 

 into the error of identifying a short interarea on each 

 valve. The error would be produced by the "gouttiere 

 cardinale" or ginglymus on the ventral valve into 

 which the cardinal margin of the dorsal valve is in- 

 serted. Such a ginglymoid joint is a not unusual 

 feature of many productids and has frequently been 

 erroneously described as a rudimentary interarea. An- 

 other feature of Productorthis is the crural fossette in 

 the inner face of the dental plates, which Kozlowski 

 interpreted correctly as a socket for the insertion of 

 the ventral edge of the brachiophore. This is the first 

 use of the term and the first recognition of the func- 

 tion of these grooves, although they are almost uni- 

 versal structures throughout the Orthacea. 



There are three interesting structures in the dorsal 

 valve. Important among these is the evident presence 

 of six muscle impressions, because of the bipartite char- 

 acter of the anterior adductors (see pi. 3, fig. 11). 

 The latter are very variable as to size and as to the pro- 

 portionate dimensions of the two components. It is 

 possible that the anterior pair represents two muscles 

 whose scars are somewhat lobate, but the definiteness 

 of the impressions favors the view that there are 

 actually four muscles represented in the anterior im- 

 pressions. To what set of muscles this extra pair 

 could be assigned is a problem. We may perhaps be 

 guided by King (1850),*° who has interpreted the 

 musculature of Productus as follows: The anterior 

 four impressions are the adductors and the scars con- 

 ventionally assigned to the posterior adductors may be 

 the "inferior pedicle muscles" which have no comple- 

 mentary scars in the ventral valve as they were at- 

 tached to the pedicle. This view would not be in 

 accord with the usual interpretation of the muscula- 

 ture of an orthid but is worthy at le.ist of some 

 consideration. 



" Op. cit., p. 6. 



** (Translation) Its suppression among the forms with 

 which we are concerned is the more important in that it 

 involves the closing of the delthyrium and the consequent 

 disappearance of the peduncle at least as a functional organ. 



"Mon. Perm. Foss., pp. 74-75 and pi. 19, fig. 4. 



