260 CEETACEOUS PELECYPODA 



1. Sippagus, Lea, 183e3, (Oontrib. to GeoL, p. 72). Shell ovate, higlier than 

 long, tumid, with prominent, attenuated, incurved beaks, of moderate thickness, 

 internal and external superficial layers of a silky appearance ; hinge edentulous, with 

 a simple insinuation or a slight notch ; ligamental furrow sub-internal, lunula not 

 excavated, muscular impressions ovate, marginal; type, H. isocardioides, Lea, 

 from tertiary beds of Alabama. 



I have already (p. 225) noticed that this shell cannot be classed wdth Verti- 

 cordia and others, as has been done by most authors. The form and silky structure 

 of the internal layer of the shell indicate a close relation to Ungulina and 

 Mysia in the same manner, as Cryptodon or Loripes is allied to typical Luclna. 

 Besides the type species I only know a second one, H. JEtnilianus, which occurs 

 in the South Indian cretaceous deposits, and is very closely allied to the American 

 form. 



2. ScaccMa, Philippi, 1844. Shell sub-orbicular, thin, muscular impressions 

 roundly ovate, sub-equal, left valve with one, right valve with two small cardinal 

 teeth, lateral teeth only indicated by very slightly raised edges of the inner anterior 

 and posterior margins near the beaks. Type, S, elUptiGa, Scacchi, from the Medi- 

 terranean ; the only other species known is aS". ovata, Phil., which is doubtfully 

 identified with the Crag Kellia cycladia of Wood. 



3. Ungulma, Daudin, 1802. Shell sub-orbicular, covered with a thick 

 wrinkled epidermis, hinge with two diverging cardinal teeth in each valve, ligament 

 sub-internal; muscular impression narrow, elongated. Type, Z7. rubra, Roissy, is 

 said to be an estuarine species from the Senegal. H. and A. Adams mention that 

 a few species live in the Mediterranean, but Weinkauf f in his '' Conchil. d. 

 Mittelmeeres" does not notice any. 



Shells which in external form resemble Ungulina as well as ScaccMa are found 

 already in the Trias and Lias, and they are referrible to either one or the other 

 genus ; some have been described as JJngulina, but it is more probable that they 

 belong to ScaccUa, as this is a purely marine form and has the ligament external. 



4. Cyrenoida, Joannis, 1835, f Cyrenella,^ Desh., 1835). Shell roundly 

 sub-quadrangular, beaks small, almost anterior, covered with a thick epidermis ; 

 muscular impressions large, ovate ; ligament external, very long ; hinge with 

 three cardinal teeth in each valve ; in the right the posterior is very small and 

 sometimes almost obsolete ; the middle is strongest, sub-triangular and bifid, the 

 anterior is thin, lamelliform ; these two last teeth are directed anteriorly ; the left 

 valve has one large lamelliform anterior cardinal, directed forward, and two very 

 small posterior ones, the last being, as in the other valve, represented only by a 

 slight thickening of the frontal edge of the fulcrum. Type, (7. Duponiii, Joan., 

 from the Senegal. 



Philippi (Handb., p. 316,) says that there is a pallial sinus present, which I 

 could not observe in the above mentioned type species. H. and A. Adams proposed 



^ This name was publislied in the same year as that of Cyrenoida, but it is printed on a subsequent page of 

 the same Volume, and has, therefore, no doubt been handed over somewhat later for publication. 



